X-PLOR has a powerful atom selection syntax that allows one to select atoms without reference to the internal index and to construct arbitrary logical expressions of selected atoms. The number of selected atoms from the last executed selection statement is stored in the symbol $SELECT.
Atom selections are generally fragile, which means that information associated with atom selections is lost when changing the molecular structure (Sections 3.7, 3.8, 3.9, 3.10, and 3.11). This applies, for example, to the various selections in the xrefin statement (Section 12.3) and in the NOE statement (Section 18.1). However, certain selections are rendered only partially fragile by mapping the selected atoms to the new molecular structure after it has been modified. This applies to all atom properties (Section 2.16) except for the internal stores, which are fragile. It also applies to the atom-based parameter statements (Section 3.2.1). Note, however, that the atom selections are not applied to any newly created atoms and these atoms are then not selected. (The reason for this is that X-PLOR does not store the strings associated with the selections.)