diff --git a/.github/workflows/typos.toml b/.github/workflows/typos.toml
index cef8a2b..471d317 100644
--- a/.github/workflows/typos.toml
+++ b/.github/workflows/typos.toml
@@ -2,3 +2,4 @@
 groth = "groth" # to avoid it dectecting it as 'growth'
 BA = "BA"
 Ded = "Ded" # "ANDed", it thought "Ded" should be "Dead"
+OT = "OT"
\ No newline at end of file
diff --git a/src/backends/plonky2/mock_main/mod.rs b/src/backends/plonky2/mock_main/mod.rs
index a708331..b557715 100644
--- a/src/backends/plonky2/mock_main/mod.rs
+++ b/src/backends/plonky2/mock_main/mod.rs
@@ -7,7 +7,7 @@ use std::fmt;
 
 use crate::middleware::{
     self, hash_str, AnchoredKey, Hash, MainPodInputs, NativeOperation, NativePredicate, NonePod,
-    Params, Pod, PodId, PodProver, StatementArg, ToFields, KEY_TYPE, SELF,
+    OperationType, Params, Pod, PodId, PodProver, StatementArg, ToFields, KEY_TYPE, SELF,
 };
 
 mod operation;
@@ -261,7 +261,11 @@ impl MockMainPod {
                 .map(|mid_arg| Self::find_op_arg(statements, mid_arg))
                 .collect::<Result<Vec<_>>>()?;
             Self::pad_operation_args(params, &mut args);
-            operations.push(Operation(op.code(), args));
+            let op_code = match op.code() {
+                OperationType::Native(code) => code,
+                _ => unimplemented!(),
+            };
+            operations.push(Operation(op_code, args));
         }
         Ok(operations)
     }
diff --git a/src/backends/plonky2/mock_main/operation.rs b/src/backends/plonky2/mock_main/operation.rs
index cb5ff3a..f5ae1de 100644
--- a/src/backends/plonky2/mock_main/operation.rs
+++ b/src/backends/plonky2/mock_main/operation.rs
@@ -2,7 +2,7 @@ use anyhow::Result;
 use std::fmt;
 
 use super::Statement;
-use crate::middleware::{self, NativeOperation};
+use crate::middleware::{self, NativeOperation, OperationType};
 
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub enum OperationArg {
@@ -29,7 +29,7 @@ impl Operation {
                 OperationArg::Index(i) => Some(statements[*i].clone().try_into()),
             })
             .collect::<Result<Vec<crate::middleware::Statement>>>()?;
-        middleware::Operation::op(self.0, &deref_args)
+        middleware::Operation::op(OperationType::Native(self.0), &deref_args)
     }
 }
 
diff --git a/src/frontend/custom.rs b/src/frontend/custom.rs
index 8a051ff..5cdba95 100644
--- a/src/frontend/custom.rs
+++ b/src/frontend/custom.rs
@@ -6,7 +6,7 @@ use crate::middleware::{
     Predicate, StatementTmpl, StatementTmplArg, ToFields, Value, F,
 };
 
-/// Argument to an statement template
+/// Argument to a statement template
 pub enum HashOrWildcardStr {
     Hash(Hash), // represents a literal key
     Wildcard(String),
diff --git a/src/frontend/mod.rs b/src/frontend/mod.rs
index 2a66070..249845f 100644
--- a/src/frontend/mod.rs
+++ b/src/frontend/mod.rs
@@ -13,6 +13,7 @@ use crate::middleware::{
     hash_str, Hash, MainPodInputs, NativeOperation, NativePredicate, Params, PodId, PodProver,
     PodSigner, SELF,
 };
+use crate::middleware::{OperationType, Predicate};
 
 mod custom;
 mod operation;
@@ -254,7 +255,7 @@ impl MainPodBuilder {
         for arg in args.iter_mut() {
             match arg {
                 OperationArg::Statement(s) => {
-                    if s.0 == NativePredicate::ValueOf {
+                    if s.0 == Predicate::Native(NativePredicate::ValueOf) {
                         st_args.push(s.1[0].clone())
                     } else {
                         panic!("Invalid statement argument.");
@@ -266,7 +267,7 @@ impl MainPodBuilder {
                     let value_of_st = self.op(
                         public,
                         Operation(
-                            NativeOperation::NewEntry,
+                            OperationType::Native(NativeOperation::NewEntry),
                             vec![OperationArg::Entry(k.clone(), v.clone())],
                         ),
                     );
@@ -291,36 +292,49 @@ impl MainPodBuilder {
 
     pub fn op(&mut self, public: bool, mut op: Operation) -> Statement {
         use NativeOperation::*;
-        let Operation(op_type, ref mut args) = op;
+        let Operation(op_type, ref mut args) = &mut op;
         // TODO: argument type checking
         let st = match op_type {
-            None => Statement(NativePredicate::None, vec![]),
-            NewEntry => Statement(NativePredicate::ValueOf, self.op_args_entries(public, args)),
-            CopyStatement => todo!(),
-            EqualFromEntries => {
-                Statement(NativePredicate::Equal, self.op_args_entries(public, args))
-            }
-            NotEqualFromEntries => Statement(
-                NativePredicate::NotEqual,
-                self.op_args_entries(public, args),
-            ),
-            GtFromEntries => Statement(NativePredicate::Gt, self.op_args_entries(public, args)),
-            LtFromEntries => Statement(NativePredicate::Lt, self.op_args_entries(public, args)),
-            TransitiveEqualFromStatements => todo!(),
-            GtToNotEqual => todo!(),
-            LtToNotEqual => todo!(),
-            ContainsFromEntries => Statement(
-                NativePredicate::Contains,
-                self.op_args_entries(public, args),
-            ),
-            NotContainsFromEntries => Statement(
-                NativePredicate::NotContains,
-                self.op_args_entries(public, args),
-            ),
-            RenameContainedBy => todo!(),
-            SumOf => todo!(),
-            ProductOf => todo!(),
-            MaxOf => todo!(),
+            OperationType::Native(o) => match o {
+                None => Statement(Predicate::Native(NativePredicate::None), vec![]),
+                NewEntry => Statement(
+                    Predicate::Native(NativePredicate::ValueOf),
+                    self.op_args_entries(public, args),
+                ),
+                CopyStatement => todo!(),
+                EqualFromEntries => Statement(
+                    Predicate::Native(NativePredicate::Equal),
+                    self.op_args_entries(public, args),
+                ),
+                NotEqualFromEntries => Statement(
+                    Predicate::Native(NativePredicate::NotEqual),
+                    self.op_args_entries(public, args),
+                ),
+                GtFromEntries => Statement(
+                    Predicate::Native(NativePredicate::Gt),
+                    self.op_args_entries(public, args),
+                ),
+                LtFromEntries => Statement(
+                    Predicate::Native(NativePredicate::Lt),
+                    self.op_args_entries(public, args),
+                ),
+                TransitiveEqualFromStatements => todo!(),
+                GtToNotEqual => todo!(),
+                LtToNotEqual => todo!(),
+                ContainsFromEntries => Statement(
+                    Predicate::Native(NativePredicate::Contains),
+                    self.op_args_entries(public, args),
+                ),
+                NotContainsFromEntries => Statement(
+                    Predicate::Native(NativePredicate::NotContains),
+                    self.op_args_entries(public, args),
+                ),
+                RenameContainedBy => todo!(),
+                SumOf => todo!(),
+                ProductOf => todo!(),
+                MaxOf => todo!(),
+            },
+            _ => todo!(),
         };
         self.operations.push(op);
         if public {
@@ -440,7 +454,7 @@ impl MainPodCompiler {
 
     fn compile_op(&self, op: &Operation) -> middleware::Operation {
         // TODO
-        let mop_code: middleware::NativeOperation = op.0.into();
+        let mop_code: OperationType = op.0.clone();
         let mop_args =
             op.1.iter()
                 .flat_map(|arg| self.compile_op_arg(arg).map(|s| s.try_into().unwrap()))
@@ -496,22 +510,22 @@ pub mod build_utils {
     #[macro_export]
     macro_rules! op {
         (eq, $($arg:expr),+) => { crate::frontend::Operation(
-            crate::middleware::NativeOperation::EqualFromEntries,
+            crate::middleware::OperationType::Native(crate::middleware::NativeOperation::EqualFromEntries),
             crate::op_args!($($arg),*)) };
         (ne, $($arg:expr),+) => { crate::frontend::Operation(
-            crate::middleware::NativeOperation::NotEqualFromEntries,
+            crate::middleware::OperationType::Native(crate::middleware::NativeOperation::NotEqualFromEntries),
             crate::op_args!($($arg),*)) };
         (gt, $($arg:expr),+) => { crate::frontend::Operation(
-            crate::middleware::NativeOperation::GtFromEntries,
+            crate::middleware::OperationType::Native(crate::middleware::NativeOperation::GtFromEntries),
             crate::op_args!($($arg),*)) };
         (lt, $($arg:expr),+) => { crate::frontend::Operation(
-            crate::middleware::NativeOperation::LtFromEntries,
+            crate::middleware::OperationType::Native(crate::middleware::NativeOperation::LtFromEntries),
             crate::op_args!($($arg),*)) };
         (contains, $($arg:expr),+) => { crate::frontend::Operation(
-            crate::middleware::NativeOperation::ContainsFromEntries,
+            crate::middleware::OperationType::Native(crate::middleware::NativeOperation::ContainsFromEntries),
             crate::op_args!($($arg),*)) };
         (not_contains, $($arg:expr),+) => { crate::frontend::Operation(
-            crate::middleware::NativeOperation::NotContainsFromEntries,
+            crate::middleware::OperationType::Native(crate::middleware::NativeOperation::NotContainsFromEntries),
             crate::op_args!($($arg),*)) };
     }
 }
diff --git a/src/frontend/operation.rs b/src/frontend/operation.rs
index 57d6f4f..ef8114f 100644
--- a/src/frontend/operation.rs
+++ b/src/frontend/operation.rs
@@ -1,7 +1,7 @@
 use std::fmt;
 
 use super::{AnchoredKey, SignedPod, Statement, StatementArg, Value};
-use crate::middleware::{hash_str, NativeOperation, NativePredicate};
+use crate::middleware::{hash_str, NativeOperation, NativePredicate, OperationType, Predicate};
 
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub enum OperationArg {
@@ -55,7 +55,7 @@ impl From<(&SignedPod, &str)> for OperationArg {
         // TODO: Actual value, TryFrom.
         let value = pod.kvs().get(&hash_str(key)).unwrap().clone();
         Self::Statement(Statement(
-            NativePredicate::ValueOf,
+            Predicate::Native(NativePredicate::ValueOf),
             vec![
                 StatementArg::Key(AnchoredKey(pod.origin(), key.to_string())),
                 StatementArg::Literal(Value::Raw(value)),
@@ -65,7 +65,7 @@ impl From<(&SignedPod, &str)> for OperationArg {
 }
 
 #[derive(Clone, Debug, PartialEq, Eq)]
-pub struct Operation(pub NativeOperation, pub Vec<OperationArg>);
+pub struct Operation(pub OperationType, pub Vec<OperationArg>);
 
 impl fmt::Display for Operation {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
diff --git a/src/frontend/statement.rs b/src/frontend/statement.rs
index 59a75e2..c0dfc25 100644
--- a/src/frontend/statement.rs
+++ b/src/frontend/statement.rs
@@ -2,7 +2,7 @@ use anyhow::{anyhow, Result};
 use std::fmt;
 
 use super::{AnchoredKey, Value};
-use crate::middleware::{self, NativePredicate};
+use crate::middleware::{self, NativePredicate, Predicate};
 
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub enum StatementArg {
@@ -20,7 +20,7 @@ impl fmt::Display for StatementArg {
 }
 
 #[derive(Clone, Debug, PartialEq, Eq)]
-pub struct Statement(pub NativePredicate, pub Vec<StatementArg>);
+pub struct Statement(pub Predicate, pub Vec<StatementArg>);
 
 impl TryFrom<Statement> for middleware::Statement {
     type Error = anyhow::Error;
@@ -33,38 +33,50 @@ impl TryFrom<Statement> for middleware::Statement {
             s.1.get(1).cloned(),
             s.1.get(2).cloned(),
         );
-        Ok(match (s.0, args) {
-            (NP::None, (None, None, None)) => MS::None,
-            (NP::ValueOf, (Some(SA::Key(ak)), Some(StatementArg::Literal(v)), None)) => {
-                MS::ValueOf(ak.into(), (&v).into())
-            }
-            (NP::Equal, (Some(SA::Key(ak1)), Some(SA::Key(ak2)), None)) => {
-                MS::Equal(ak1.into(), ak2.into())
-            }
-            (NP::NotEqual, (Some(SA::Key(ak1)), Some(SA::Key(ak2)), None)) => {
-                MS::NotEqual(ak1.into(), ak2.into())
-            }
-            (NP::Gt, (Some(SA::Key(ak1)), Some(SA::Key(ak2)), None)) => {
-                MS::Gt(ak1.into(), ak2.into())
-            }
-            (NP::Lt, (Some(SA::Key(ak1)), Some(SA::Key(ak2)), None)) => {
-                MS::Lt(ak1.into(), ak2.into())
-            }
-            (NP::Contains, (Some(SA::Key(ak1)), Some(SA::Key(ak2)), None)) => {
-                MS::Contains(ak1.into(), ak2.into())
-            }
-            (NP::NotContains, (Some(SA::Key(ak1)), Some(SA::Key(ak2)), None)) => {
-                MS::NotContains(ak1.into(), ak2.into())
-            }
-            (NP::SumOf, (Some(SA::Key(ak1)), Some(SA::Key(ak2)), Some(SA::Key(ak3)))) => {
-                MS::SumOf(ak1.into(), ak2.into(), ak3.into())
-            }
-            (NP::ProductOf, (Some(SA::Key(ak1)), Some(SA::Key(ak2)), Some(SA::Key(ak3)))) => {
-                MS::ProductOf(ak1.into(), ak2.into(), ak3.into())
-            }
-            (NP::MaxOf, (Some(SA::Key(ak1)), Some(SA::Key(ak2)), Some(SA::Key(ak3)))) => {
-                MS::MaxOf(ak1.into(), ak2.into(), ak3.into())
-            }
+        Ok(match &s.0 {
+            Predicate::Native(np) => match (np, args) {
+                (NP::None, (None, None, None)) => MS::None,
+                (NP::ValueOf, (Some(SA::Key(ak)), Some(StatementArg::Literal(v)), None)) => {
+                    MS::ValueOf(ak.into(), (&v).into())
+                }
+                (NP::Equal, (Some(SA::Key(ak1)), Some(SA::Key(ak2)), None)) => {
+                    MS::Equal(ak1.into(), ak2.into())
+                }
+                (NP::NotEqual, (Some(SA::Key(ak1)), Some(SA::Key(ak2)), None)) => {
+                    MS::NotEqual(ak1.into(), ak2.into())
+                }
+                (NP::Gt, (Some(SA::Key(ak1)), Some(SA::Key(ak2)), None)) => {
+                    MS::Gt(ak1.into(), ak2.into())
+                }
+                (NP::Lt, (Some(SA::Key(ak1)), Some(SA::Key(ak2)), None)) => {
+                    MS::Lt(ak1.into(), ak2.into())
+                }
+                (NP::Contains, (Some(SA::Key(ak1)), Some(SA::Key(ak2)), None)) => {
+                    MS::Contains(ak1.into(), ak2.into())
+                }
+                (NP::NotContains, (Some(SA::Key(ak1)), Some(SA::Key(ak2)), None)) => {
+                    MS::NotContains(ak1.into(), ak2.into())
+                }
+                (NP::SumOf, (Some(SA::Key(ak1)), Some(SA::Key(ak2)), Some(SA::Key(ak3)))) => {
+                    MS::SumOf(ak1.into(), ak2.into(), ak3.into())
+                }
+                (NP::ProductOf, (Some(SA::Key(ak1)), Some(SA::Key(ak2)), Some(SA::Key(ak3)))) => {
+                    MS::ProductOf(ak1.into(), ak2.into(), ak3.into())
+                }
+                (NP::MaxOf, (Some(SA::Key(ak1)), Some(SA::Key(ak2)), Some(SA::Key(ak3)))) => {
+                    MS::MaxOf(ak1.into(), ak2.into(), ak3.into())
+                }
+                _ => Err(anyhow!("Ill-formed statement: {}", s))?,
+            },
+            Predicate::Custom(cpr) => MS::Custom(
+                cpr.clone(),
+                s.1.iter()
+                    .map(|arg| match arg {
+                        StatementArg::Key(ak) => Ok(ak.clone().into()),
+                        _ => Err(anyhow!("Invalid statement arg: {}", arg)),
+                    })
+                    .collect::<Result<Vec<_>>>()?,
+            ),
             _ => Err(anyhow!("Ill-formed statement: {}", s))?,
         })
     }
diff --git a/src/middleware/operation.rs b/src/middleware/operation.rs
index cf08705..dd22cc3 100644
--- a/src/middleware/operation.rs
+++ b/src/middleware/operation.rs
@@ -8,6 +8,12 @@ use crate::{
     util::hashmap_insert_no_dupe,
 };
 
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub enum OperationType {
+    Native(NativeOperation),
+    Custom(CustomPredicateRef),
+}
+
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum NativeOperation {
     None = 0,
@@ -51,26 +57,27 @@ pub enum Operation {
 }
 
 impl Operation {
-    pub fn code(&self) -> NativeOperation {
+    pub fn code(&self) -> OperationType {
+        type OT = OperationType;
         use NativeOperation::*;
         match self {
-            Self::None => None,
-            Self::NewEntry => NewEntry,
-            Self::CopyStatement(_) => CopyStatement,
-            Self::EqualFromEntries(_, _) => EqualFromEntries,
-            Self::NotEqualFromEntries(_, _) => NotEqualFromEntries,
-            Self::GtFromEntries(_, _) => GtFromEntries,
-            Self::LtFromEntries(_, _) => LtFromEntries,
-            Self::TransitiveEqualFromStatements(_, _) => TransitiveEqualFromStatements,
-            Self::GtToNotEqual(_) => GtToNotEqual,
-            Self::LtToNotEqual(_) => LtToNotEqual,
-            Self::ContainsFromEntries(_, _) => ContainsFromEntries,
-            Self::NotContainsFromEntries(_, _) => NotContainsFromEntries,
-            Self::RenameContainedBy(_, _) => RenameContainedBy,
-            Self::SumOf(_, _, _) => SumOf,
-            Self::ProductOf(_, _, _) => ProductOf,
-            Self::MaxOf(_, _, _) => MaxOf,
-            Self::Custom(_, _) => todo!(),
+            Self::None => OT::Native(None),
+            Self::NewEntry => OT::Native(NewEntry),
+            Self::CopyStatement(_) => OT::Native(CopyStatement),
+            Self::EqualFromEntries(_, _) => OT::Native(EqualFromEntries),
+            Self::NotEqualFromEntries(_, _) => OT::Native(NotEqualFromEntries),
+            Self::GtFromEntries(_, _) => OT::Native(GtFromEntries),
+            Self::LtFromEntries(_, _) => OT::Native(LtFromEntries),
+            Self::TransitiveEqualFromStatements(_, _) => OT::Native(TransitiveEqualFromStatements),
+            Self::GtToNotEqual(_) => OT::Native(GtToNotEqual),
+            Self::LtToNotEqual(_) => OT::Native(LtToNotEqual),
+            Self::ContainsFromEntries(_, _) => OT::Native(ContainsFromEntries),
+            Self::NotContainsFromEntries(_, _) => OT::Native(NotContainsFromEntries),
+            Self::RenameContainedBy(_, _) => OT::Native(RenameContainedBy),
+            Self::SumOf(_, _, _) => OT::Native(SumOf),
+            Self::ProductOf(_, _, _) => OT::Native(ProductOf),
+            Self::MaxOf(_, _, _) => OT::Native(MaxOf),
+            Self::Custom(cpr, _) => OT::Custom(cpr.clone()),
         }
     }
 
@@ -96,40 +103,45 @@ impl Operation {
         }
     }
     /// Forms operation from op-code and arguments.
-    pub fn op(op_code: NativeOperation, args: &[Statement]) -> Result<Self> {
+    pub fn op(op_code: OperationType, args: &[Statement]) -> Result<Self> {
         type NO = NativeOperation;
         let arg_tup = (
             args.get(0).cloned(),
             args.get(1).cloned(),
             args.get(2).cloned(),
         );
-        Ok(match (op_code, arg_tup, args.len()) {
-            (NO::None, (None, None, None), 0) => Self::None,
-            (NO::NewEntry, (None, None, None), 0) => Self::NewEntry,
-            (NO::CopyStatement, (Some(s), None, None), 1) => Self::CopyStatement(s),
-            (NO::EqualFromEntries, (Some(s1), Some(s2), None), 2) => Self::EqualFromEntries(s1, s2),
-            (NO::NotEqualFromEntries, (Some(s1), Some(s2), None), 2) => {
-                Self::NotEqualFromEntries(s1, s2)
-            }
-            (NO::GtFromEntries, (Some(s1), Some(s2), None), 2) => Self::GtFromEntries(s1, s2),
-            (NO::LtFromEntries, (Some(s1), Some(s2), None), 2) => Self::LtFromEntries(s1, s2),
-            (NO::ContainsFromEntries, (Some(s1), Some(s2), None), 2) => {
-                Self::ContainsFromEntries(s1, s2)
-            }
-            (NO::NotContainsFromEntries, (Some(s1), Some(s2), None), 2) => {
-                Self::NotContainsFromEntries(s1, s2)
-            }
-            (NO::RenameContainedBy, (Some(s1), Some(s2), None), 2) => {
-                Self::RenameContainedBy(s1, s2)
-            }
-            (NO::SumOf, (Some(s1), Some(s2), Some(s3)), 3) => Self::SumOf(s1, s2, s3),
-            (NO::ProductOf, (Some(s1), Some(s2), Some(s3)), 3) => Self::ProductOf(s1, s2, s3),
-            (NO::MaxOf, (Some(s1), Some(s2), Some(s3)), 3) => Self::MaxOf(s1, s2, s3),
-            _ => Err(anyhow!(
-                "Ill-formed operation {:?} with arguments {:?}.",
-                op_code,
-                args
-            ))?,
+        Ok(match op_code {
+            OperationType::Native(o) => match (o, arg_tup, args.len()) {
+                (NO::None, (None, None, None), 0) => Self::None,
+                (NO::NewEntry, (None, None, None), 0) => Self::NewEntry,
+                (NO::CopyStatement, (Some(s), None, None), 1) => Self::CopyStatement(s),
+                (NO::EqualFromEntries, (Some(s1), Some(s2), None), 2) => {
+                    Self::EqualFromEntries(s1, s2)
+                }
+                (NO::NotEqualFromEntries, (Some(s1), Some(s2), None), 2) => {
+                    Self::NotEqualFromEntries(s1, s2)
+                }
+                (NO::GtFromEntries, (Some(s1), Some(s2), None), 2) => Self::GtFromEntries(s1, s2),
+                (NO::LtFromEntries, (Some(s1), Some(s2), None), 2) => Self::LtFromEntries(s1, s2),
+                (NO::ContainsFromEntries, (Some(s1), Some(s2), None), 2) => {
+                    Self::ContainsFromEntries(s1, s2)
+                }
+                (NO::NotContainsFromEntries, (Some(s1), Some(s2), None), 2) => {
+                    Self::NotContainsFromEntries(s1, s2)
+                }
+                (NO::RenameContainedBy, (Some(s1), Some(s2), None), 2) => {
+                    Self::RenameContainedBy(s1, s2)
+                }
+                (NO::SumOf, (Some(s1), Some(s2), Some(s3)), 3) => Self::SumOf(s1, s2, s3),
+                (NO::ProductOf, (Some(s1), Some(s2), Some(s3)), 3) => Self::ProductOf(s1, s2, s3),
+                (NO::MaxOf, (Some(s1), Some(s2), Some(s3)), 3) => Self::MaxOf(s1, s2, s3),
+                _ => Err(anyhow!(
+                    "Ill-formed operation {:?} with arguments {:?}.",
+                    op_code,
+                    args
+                ))?,
+            },
+            OperationType::Custom(cpr) => Self::Custom(cpr, args.to_vec()),
         })
     }
     /// Checks the given operation against a statement.