DO* ({(Var [Init] [Step])}) (Test Exit-Form) Declaration* Form*
Iteration construct. Each Var is initialized sequentially (like LET*) to the
value of the specified Init form. On subsequent iterations, the Vars are
sequentially assigned the value of the Step form (if any). The Test is
evaluated before each evaluation of the body Forms. When the Test is true,
the Exit-Forms are evaluated as a PROGN, with the result being the value
of the DO. A block named NIL is established around the entire expansion,
allowing RETURN to be used as an alternate exit mechanism.