NAME:
    prival
 PURPOSE: (one line)
    Reduce an angle to its principal value (0 $\leq \Theta < 2\pi$).
 DESCRIPTION:
    This routine uses [0 to 2*PI) as the principal value of an angle.  If
 the angle is negative, 2*PI is added to the remainder of division by 2*PI.
    If positive, then the value is the remainder of division by 2*PI.  Nothing
 is done to the input value if it is already in range.

 CATEGORY:
    Mathematical
 CALLING SEQUENCE:
    result = prival( angle )
 INPUTS:
    angle : Angle to reduce to principal value (double).
 OPTIONAL INPUT PARAMETERS:

 KEYWORD PARAMETERS:

 OUTPUTS:
    result = Principal value of angle (double).
 COMMON BLOCKS:

 SIDE EFFECTS:

 RESTRICTIONS:

 PROCEDURE:

 MODIFICATION HISTORY:
    Copyright (C) 1987, by Marc W. Buie
    Version dated 86/2/1
    Ported by Doug Loucks, Lowell Observatory, July 28, 1993, from the
 C-language version written by Marc Buie.