Program BEND, Module BnnC.for

                           SUBROUTINE STEP2

Construct the guiding strip (not developable) by twisting the rectifying
developable about the base curve, which is held fixed.

..............................................................................

     IF ( NOT (Option 7)  AND  NOT guessing A-length)  THEN
        WRITE the total twist of the developable strip.

2000 Enter the twist loop.
     IF ( NOT SILENT)  THEN
        WRITE the final edge angle of the strip.
        READ  the preferred angle.
     END_IF
     IF ( guessing A-length OR SILENT) GO TO 1100

     WRITE total twist of the current strip.
     WRITE current values of the twist parameters: EXPAND, SHIFT, BLUNT, ...
     READ  preferred values of the twist parameters.
     WRITE new values of the twist parameters.

1100 Set the indices of the two points on the base curve at which the
     splining of the additional-twist of the rectifying developable starts
     and ends, to be equal to the indices of the first (namely, 1) and
     last points on the base curve, respectively.
     DO the additional-twist, moving the starting and ending of its
        splining towards each other until the new free edge of the strip
        does not bulge outside of the bounds given by its prescribed
        values at the first and last points on the base curve.
        CALL SPLINE to smoothly interpolate the additional-twist angle
                    between the current starting and ending points.
        CALL TORSE  to twist the strip away from the rectifying devel-
                    opable by the additional-twist angle amount.
        Compute the lateral and axial bulges.
        IF (neither bulge is greater than zero)  GO TO 1121
     END_DO the additional-twist

1121 IF (the start of the spline has been moved up  AND  NOT SILENT)  THEN
        WRITE the index of the twist-spline starting point.
     IF (the end of the spline has been moved back  AND  NOT SILENT)  THEN
        WRITE the index of the twist-spline ending point.

     IF (bluntness) THEN spread (or contract) the strip by another twist
        about its base curve.

9600 Compute the rate of twist of the strip.

     IF ( NOT SILENT) WRITE the overall (delta L)/L.

     IF ( NOT guessing the A-length  AND  NOT SILENT)  THEN
        WRITE for a selected subset of points, including those at which
           maxima or minima are indicated by asterisks, the values of:
              Point number
              Arc length along the base curve to this point
              (Delta L) / L
              Additional-twist angle
              Radius of curvature of the base curve
              Radius of curvature of the free edge
              Twist rate
     END_IF

     WRITE the index-numbers of the points on the free edge which have
           been bulged out of the bounds prescribed by its first and
           last points.

     IF ( SILENT  OR  Option 7  OR  guessing A-length)  GO TO 9999
     IF ( NOT DEBUG)  GO TO 9830

     WRITE "Press RETURN to continue ..."

     WRITE for a selected subset of points, including those at which
        maxima or minima are indicated by asterisks, the values of:
           Point number
           Arc length along the base curve to this point
           (delta L)/L
           Additional-twist angle
           Angle between the strip and the normal to the cylinder
           Strain-energy density of the base curve
           Strain-energy density of the free edge

9850 WRITE "Do you want to try different values of shift, bluntness, etc.?"
     IF (yes)  GO TO 2000
     ELSE exit from the twist loop.

9999 RETURN

     END

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