NOT11 — Constraint Language issue

Summary: Because the constraint language is inherited from the Trader specification, we
have these categories:

<expr_in> := <expr_twiddle> in <Ident> | <expr_twiddle>
<expr_twiddle> := <expr> ~ <expr> | <expr>

It seems that the point of the "in" operator is to determine whether the LHE
is in the RHE where the RHE is a sequence. However, there"s no way to access
the current event since the RHE is only allowed to be an Ident, not "$"
<Component>. It appears that the RHE could only be an unstructured runtime
variable in practice. If this is not the case, how is it supposed to work?

In addition, the <preference> syntactic category makes no sense. I assume that
it is not be supported. This is perhaps a small point, but I can"t find a
reference to it.