If one 
-expression reduces to another, we say they are equivalent
Some -expressions are irreducible (x.(x y) for example)
If expression E is equivalent to irreducible F, F is a normal form for E
There might be several ways to reduce an expression:
(
x.p x)((y.b) q)(p ((
y.b) q))(p b)
Or
(
x.p x)((y.b) q)(
x.p x) b(p b)
