If s is the name of the current theory or an ancestor of the
current theory, the call parents s returns a list of strings that
identify the parent theories of s. The shorthand "-" may be
used to denote the name of the current theory segment.
FAILURE
Fails if the named theory is not an ancestor of the current theory.
EXAMPLE
- parents "bool";
> val it = ["min"] : string list
- parents "min";
> val it = [] : string list
- current_theory();
> val it = "scratch" : string
- parents "-";
> val it = ["list", "option"] : string list