The function binders should be applied to a string which is the name of an
ancestor theory (including the current theory; the special string "-" is
always interpreted as the current theory). It returns a list of all the binders
declared in the named theory.
FAILURE
Fails unless the given theory is an ancestor of the current theory.
EXAMPLE
- binders "bool";
val it = [`$?!`, `$!`, `$@`] : term list
- binders "prod";
val it = [] : term list