Note that criterion 2 eliminates any atom type specifications that violate valence restrictions, such as a carbon with more than four bonds. All of these criteria are applied when the molecules are typed with the full chemical perception tree, so criteria 2 and 4 will only hold if types A and A' are both assigned to at least one atom. If these criteria are met, the proposed atom type set is valid and can be scored (Section 2.1.1).