Some selections take no values. For example, backbone selects the backbone atoms of the protein and nucleic acids and protein selects protein atoms. Giving options to these selections is an error. The selections can be used in the same way as other selections, as in:
protein and backbone
nucleic or protein
In addition, if neither and nor or are located
after a boolean keyword, then an implicit and is inserted, so
that the following are valid:
protein name CA (same as: protein and name CA)
nucleic backbone