去年小学信息学奥赛的一道题,我怎么也看不懂,请帮忙分析一下,万分感谢
DECLARE FUNCTION c (a AS INTEGER, b AS INTEGER)
  PRINT c(6, 4)

  FUNCTION c (a AS INTEGER, b AS INTEGER)
    IF ((a = b) OR (b = 0)) THEN
     c = 1
    ELSE
     c = c(a - 1, b - 1) - c(a - 1, b)
    END IF
  END FUNCTION
  运行结果为：____________________
结果是7,过程倒是看的懂,但c = c(a - 1, b - 1) - c(a - 1, b) 这语句说什么也不理解,也就不知道怎么得的7,请帮忙具体说明一下.