erf() returns the value of the error function. The definition of
    the erf() function is (2/sqrt(pi)) times the area under the curve
    exp(-t * t) between 0 and x.

    erfc() returns (1.0-erf(x)).

                           Entry-Point Names

    Generic                               Compaq
    Function   Data Type   OpenVMS        Tru64 UNIX
    Name       Required    Alpha          Alpha

    erf        S_FLOAT     math$erf_s     erff
               T_FLOAT     math$erf_t     erf
               X_FLOAT     math$erf_x     erfl
               F_FLOAT     math$erf_f
               G_FLOAT     math$erf_g

    erfc       S_FLOAT     math$erfc_s    erfcf
               T_FLOAT     math$erfc_t    erfc
               X_FLOAT     math$erfc_x    erfcl
               F_FLOAT     math$erfc_f
               G_FLOAT     math$erfc_g