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.