mk_selectboolSyntax.mk_select : term * term -> term
Constructs a choice-term.
If v is a variable and t is a term of type
bool, then mk_select (v,t) returns
@var. t.
Fails if v is not a variable or if t is not
of type bool.
boolSyntax.dest_select,
boolSyntax.is_select