2.4.17 The PUSH Instruction PUSH [ expr ] ; The PUSH instruction will add a string to the stack. The string added will either be the result of the expr, or the nullstring if expr is not specified. The string will be added to the top of the stack (LIFO), i.e. it will be the first line normally extracted from the stack. For a thorough discussion of the stack and the methods of manipulating it, see chapter Stack for a discussion of the stack.