A program called BFIELD.F is distributed with YAUP.F. BFIELD.F is a
standalone module that generates a B-field file for an undulator with
a linearly tapered gap in a format acceptable to YAUP.  The user must
specify the gap GZMIN at Z=Zmin=0 and the amount of gap taper
DG=(GZMAX-GZMIN)/GZMIN (either positive or negative).  The program
interpolates the gap linearly between GZMIN and GZMAX.  The field
strength dependence is specific to the proposed APS undulators but the
user may modify this section of the program according to his/her
specific needs.  Zero phase errors are assumed. See the file BFIELD.F
for more details.