Throw an error when if <test> yields a false value in (if <test> <consequent>).

4.1.5  Conditionals: If <test> yields a false value and no <alternate> is
specified, then the result of the expression is unspecified.

http://www.schemers.org/Documents/Standards/R5RS/HTML/r5rs-Z-H-7.html#%_sec_4.1.5