Example of a modification. This example describes methylation at the O1 position of pyranoses. See Fig. 2(a) for a graphical representation of this modification. The first (general) category (_chem_mod) reports the code for the modification `O1MET' and describes whether the modification is to be applied to only a particular monomer or to a group of monomers. The `O1MET' modification can be applied to all monomers belonging to the `pyranose' group. The (_chem_mod_atom) category describes the list of all added, deleted or changed atoms. The following category (_chem_mod_bond) describes all added or deleted bonds. In a similar manner, the tree structure (_chem_mod_tree), bond angles (_chem_mod_angle), torsion angles (_chem_mod_tor), planes (_chem_mod_plane_atom) and chiralities (_chem_mod_chir) affected by the modification are handled.