GammaError = enum
  geOk, geDom = "x in {0, -1, -2, ...}", ## when .
                                          ## Infinity discontinuity,
                                          ## which shall produce `Complex Infinity` in SymPy and
                                          ## means domain error
  geOverFlow = "x > MAX_GAMMA_X, result overflow as inf",
  geUnderFlow = "x < MIN_GAMMA_X, result underflow as `-0.0` or `0.0`.", geZeroCantDetSign = "`x < -maxSafeInteger`(exclude -inf), " &
      "can\'t detect result\'s sign", ## `x` losing unit digit, often not regard as an error
  geGotNegInf = "x == -inf" ## this is made as a enumerate item as different languages'
                            ## implementation has different treatment towards -inf