Problem Statement

(Given an equal-probability generator of one of the integers 1 to 5 as dice5),   create dice7 that generates a pseudo-random integer from 1 to 7 in equal probability using only dice5 as a source of random numbers,   and check the distribution for at least one million calls using the function created in   Simple Random Distribution Checker.

Implementation suggestion: dice7 might call dice5 twice, re-call if four of the 25 combinations are given, otherwise split the other 21 combinations into 7 groups of three, and return the group index from the rolls. (Task adapted from an answer here)

Let's start with the solution:

(defun d5 ()
  (1+ (random 5)))

(defun d7 ()
  (loop for d55 = (+ (* 5 (d5)) (d5) -6)
        until (< d55 21)
        finally (return (1+ (mod d55 7)))))