ENH: Indexing notation for C-sets.

Syntactic sugar for the `subpart` function.

src/categorical_algebra/CSetDataStructures.jl

@@ -329,13 +329,24 @@ end
 """ Get subpart of part in C-set.
 
 Both single and vectorized access are supported. Chaining, or composition, of
-parts is also supported. For example, given a vertex-attributed graph `g`,
+subparts is also supported. For example, given a vertex-attributed graph `g`,
 
 ```
 subpart(g, e, [:src, :vattr])
 ```
 
-returns the vertex attribute of the source vertex of the edge `e`.
+returns the vertex attribute of the source vertex of the edge `e`. As a
+shorthand, subparts can also be accessed by indexing:
+
+```
+g[e, :src] == subpart(g, e, :src)
+```
+
+Be warned that indexing with lists of subparts works as above:
+`g[e,[:src,:vattr]]` is equivalent to `subpart(g, e, [:src,:vattr])`. This
+differs from DataFrames but note that the alternative interpretation of
+`[:src,:vattr]` as two independent columns does not even make sense, since they
+have different domains (belong to different tables).
 """
 subpart(acs::ACSet, part, name::Symbol) = subpart(acs,name)[part]
 subpart(acs::ACSet, name::Symbol) = _subpart(acs,Val(name))
@@ -362,6 +373,8 @@ subpart_name(expr::GATExpr{:compose}) = mapreduce(subpart_name, vcat, args(expr)
   end
 end
 
+Base.getindex(acs::ACSet, args...) = subpart(acs, args...)
+
 """ Get superparts incident to part in C-set.
 
 If the subpart is indexed, this takes constant time; otherwise, it takes linear

test/categorical_algebra/CSetDataStructures.jl

@@ -159,6 +159,11 @@ X, parent, height = TheoryDendrogram[[:X, :parent, :height]]
 @test subpart(d, 3, id(X)) == 3
 @test incident(d, 10, compose(parent, height)) == [1,2,3]
 
+# Indexing syntax.
+@test d[3, :parent] == 4
+@test d[3, [:parent, :height]] == 10
+@test d[:, :parent] == [4,4,4,5,5]
+
 # Copying parts.
 d2 = Dendrogram{Int}()
 copy_parts!(d2, d, X=[4,5])