CLEAR removes (clears) all the atoms from the current selection.
ENTER pushed the current selection into the stack, and initializes the top of the stack to the default value.
OR performs a logical OR operation between the top of the stack and next deepest selection, pops the stack twice, and pushes the result onto the stack.
AND does the same thing as OR except for performing and AND operation.
NOT takes the inverse of the top of the stack.
EXCL is equivalent to NOT AND. In other words, it deletes all the selected atoms from the previous selection on the stack. This operation is provided for simplifying conversions from the previous form of the atom selection syntax.
EXCH i exchanges the top of the stack with the ith deepest element. The top of the stack is numbered 0.
SHOW will print the atoms that are currently included at the point in string parsing where SHOW is encountered. If you want the list that will be returned to the calling routine, make sure SHOW is the last entry in the string.