  The equations for calculating diffraction profiles for bent crystals are
revisited for both meridional and sagittal bending. Two approximated methods
for computing diffraction profiles are treated: multilamellar and
Penning-Polder. A common treatment of crystal anisotropy is included in these
models. The formulation presented is implemented into the XOP package,
completing and updating the crystal module that simulates diffraction profiles
for perfect, mosaic and now distorted crystals by elastic bending.
