An atom to be considered for pruning may have a non-zero charge. If so, subvalence is computed using the isoelectronic element's default valences. For example, a selected nitrogen atom with a charge of +1 would use the default valence for carbon. A selected phosphorous atom with a charge of -1 would use the default valences of sulfur. And so on. If no default valences are found in this way (e.g., "[c+2]"), a reader must report an error. Writers must not encode such atoms.