An occurrence of v inside E is said to be bound
if it occurs in v.E, and is free otherwise.
examples:
v occurs free in v(x y v)
v occurs bound in v.v(x y v)
v occurs both bound and free in v(x (v.y v) y)
We say v is free in E if there is at least one free occurrence of v in E.
fv(x) = {x}
fv(E1 E2) = fv(E1) 1#1
fv(E2)
fv(x.E) = fv(E) - {x}.
E is said to be closed iff fv(E) = {}.
example: z.(
x.z x)(
y.y z) is closed.
Expressions equal up to renaming are called equivalent.