if --Command or operation, two or three inputs.
     The first input to the if procedure must be either the word true or
     the word false. Typically, it is the output from a predicate. The
     second and (optional) third inputs are lists containing instruction
     lines. The second input is executed if the first input is true. The
     third input, if any, is executed if the first input is false:

     to greet :person
     if equalp :person [Ronald Reagan] [print [Hi, turkey!]] \
                         [print sentence "Hi, :person]
     end

     In that example, the first input to if is the output from the
     expression
     equalp :person [Ronald Reagan].

     The if procedure can be used as an operation, producing a value. In
     this case, the third input is required:

     print if equalp :person "Reagan ["Loser] ["Winner]