Check whether a global variable in state has a value
(f-boundp-global 'y state)
(f-boundp-global s state)
where s evaluates to a symbol. Note that this executes more
efficiently when s is a constant of the form 'sym.
This value returned is t or nil according to whether a value is
assigned to the given symbol in the ACL2 state. Also see related
utilities f-get-global, f-put-global, and makunbound-global.