Add the PA rule from Liz Coppock's Semantics Boot Camp

This is very similar to the existing lambda notebook PA rule, but works
completely syntactically and so is even simpler. It's designed to work
in a presuppositional composition system.  To swap this in, you can use:

    lang.set_system(lang.td_presup)
    lang.get_system().remove_rule("PA")
    lang.get_system().add_rule(lang.BinaryCompositionOp("PA", lang.sbc_pa,
allow_none=True))

Author: rawlins

lamb/lang.py

@@ -2509,7 +2509,16 @@ def presup_pa(binder, content, assignment=None):
     f = meta.LFun(types.type_e, content.content.calculate_partiality({bound_var}).under_assignment(new_a), outer_vname)
     return BinaryComposite(binder, content, f)
 
-
+# this is a presuppositional PA based on Liz Coppock's Semantics Boot Camp
+# PA rule.
+def sbc_pa(binder, content, assignment=None):
+    if (binder.content is not None) or not binder.name.strip().isnumeric():
+        raise TypeMismatch(binder, content, "Predicate Abstraction")
+    index = int(binder.name.strip())
+    vname = "var%i" % index
+    bound_var = meta.term(vname, types.type_e)
+    f = meta.LFun(types.type_e, content.content.calculate_partiality({bound_var}), vname)
+    return BinaryComposite(binder, content, f)
 
 
 def setup_type_driven():