(if (condition)                  # If the condition evaluates to non-NIL
   (then-do-this)                # Then execute the following expression
   (else-do-that)                # Else execute all other expressions
   (and-more) )

(ifn (condition)                 # If the condition evaluates to NIL
   (then-do-this)                # Then execute the following expression
   (else-do-that)                # Else execute all other expressions
   (and-more) )