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.