Calculates the smallest value for which a function
evaluates to be greater than or equal to
y -- that is,
the left inverse of
y. Intended for internal use only.
directional_inverse(distribution, p, low, high, tol, maxiter, direction)
A distribution having access to a cdf.
Single value for which to calculate the left inverse.
Single numeric values forming a range within which to search for the solution.
Tolerance (a small positive number) and maximum number of iterations
"left" for calculating left-inverse, or
"right" for calculating right-inverse.
This algorithm works by progressively cutting the specified range in half, so that the width of the range after k iterations is 1/2^k times the original width.