ibeta(x, a, b [, /complement])
Returns the incomplete beta function: integral over
t from 0 to
t^(a-1) (1 - t)^(b-1) /
x between 0 and 1 (inclusive)
b positive. If keyword
/complement is specified, then one minus this value is returned.
Minus one is returned for illegal parameter combinations.
See also: beta