defined and introduced variables

[MaxOstrowski]

Check out example:
tests/mzn/bool_clause_imp_1.mzn
with -O0

var bool: X_INTRODUCED_0_ ::var_is_introduced :: is_defined_var;
constraint bool_clause([X_INTRODUCED_0_],[r]);
constraint bool_clause([a1,a2,a3,a4],[a5,a6,a7,a8,a9,X_INTRODUCED_0_]):: defines_var(X_INTRODUCED_0_);

and -O2

constraint bool_clause([a4,a3,a2,a1],[r,a9,a8,a7,a6,a5]);

It seems that minizinc is allowed to introduce symmetries into the solutions.
Maybe we can avoid them somehow.