code: function gratioc(a) {
        if (a <= 0) { return function(x) { return NaN; }; }
        if (a === 0.5) { return function(x) { return erfc(Math.sqrt(x)); }; }
        if (a < 1) { return smallQ(a); }
        if (a < BIG) { return mediumQ(a); }
        return bigQ(a);
     }
docs: Returns the complement of the incomplete gamma ratio:
  $$Q(a,x) = \frac{1}{\Gamma(a)} \int_x^\infty e^{-t} t^{a-1} dt$$
  
  `a` is the shape parameter and $0 < x < \infty$.
  
  See also: `pgamma`
  
  Based on:  *Computation of the Incomplete Gamma Function Ratios
  and their Inverse*, by DiDonato and Morris, 1986
  @fullName gratioc(a)(x)
  @memberof basicFunc