b* conditional execution operator.
Typical example: this always returns
(b* ((ans (some-computation arg1 ... argn)) ((run-when (< ans 0)) (cw "Warning: answer was negative?~%") (cw "Args were ~x0, ~x1, ..." arg1 arg2 ...))) ans)
(b* (((run-when (condition-form)) (run-form1) ... (run-formn))) (result-form))
is equivalent to
(prog2$ (and (condition-form) (progn$ (run-form1) ... (run-formn))) (result-form))