The `ibeta(p,q,x)` function returns the incomplete beta function of the real parts of its arguments. p, q > 0 and x in [0:1]. If the arguments are complex, the imaginary components are ignored. The function is approximated by the method of continued fractions (Abramowitz and Stegun, 1964). The approximation is only accurate in the region x < (p-1)/(p+q-2).