Syntax:
    "if e1 then e2 else e3"
  where "if", "then", and "else" are keywords and
  e1, e2, e3 are subexpressions

Type-checking:
   1. e1 must have type bool
   2. e2 and e3 can have any type (let's call it t),
      but they must have the same type t
   3. the type of the entire expression is also t

Evaluation rules:
   1. evaluate e1 to a value v1
   2. if v1 is true, evaluate e2 and that result is
      the whole expression's result
   3. else, evaluate e3 and that result is
      the whole expression's result