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diff --git a/Dragon/src/XELPSC.f b/Dragon/src/XELPSC.f
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+*DECK XELPSC
+      FUNCTION XELPSC(RANN,PLANE)
+*
+*-----------------------------------------------------------------------
+*
+*Purpose:
+* Compute annular surface below Cartesian plane.
+*
+*Copyright:
+* Copyright (C) 1997 Ecole Polytechnique de Montreal
+* This library is free software; you can redistribute it and/or
+* modify it under the terms of the GNU Lesser General Public
+* License as published by the Free Software Foundation; either
+* version 2.1 of the License, or (at your option) any later version
+*
+*Author(s):G. Marleau
+*
+*Parameters: input
+* RANN    annular radius.
+* PLANE   Cartesian plane location.
+*
+*Parameters: output
+* XELPSC  annular surface below plane.
+*
+*-----------------------------------------------------------------------
+*
+      IMPLICIT          NONE
+      DOUBLE PRECISION  XELPSC,RANN,PLANE
+*----
+*  LOCAL VARIABLES
+*----
+      DOUBLE PRECISION  RANN2,PLANE2,ALPHA
+*----
+*  METHOD
+*  1) FIND HALF-ANGLE COVERED BY THE TWO INTERSECTION POINTS
+*     BETWEEN PLANE AND ANNULAR REGION
+*     -- ALPHA=ACOS(-PLANE/RANN)
+*  2) COMPUTED ANNULAR SURFACE COVERED BY THIS HALF-ANGLE
+*     -- 0.5*RANN2*ALPHA
+*  3) ADD SURFACE COVERED BY INTERNAL RECTANGLE IN THIS HALF-ANGLE
+*     -- 0.5*PLANE*SQRT(RANN2-PLANE2)
+*  4) DOULBLE SURFACE FOR FULL ANGLE
+*----
+      RANN2=RANN*RANN
+      PLANE2=PLANE*PLANE
+      ALPHA=ACOS(-PLANE/RANN)
+      XELPSC=RANN2*ALPHA+PLANE*SQRT(RANN2-PLANE2)
+      RETURN
+      END