2 Some authors reserve the term "term" for just variables and abstracts. We'll probably just say "term" and "expression" indiscriminately for expressions of any of these three forms.
4 Examples of expressions:
13 ((\x (x x)) (\x (x x)))
15 The lambda calculus has an associated proof theory. For now, we can regard the
16 proof theory as having just one rule, called the rule of **beta-reduction** or
17 "beta-contraction". Suppose you have some expression of the form: