A keyword instruction is one or more clauses, the first of which starts with a keyword that identifies the instruction. Keyword instructions control, for example, the external interfaces and the flow of control. Some keyword instructions can include nested instructions. In the following example, the DO construct (DO, the group of instructions that follow it, and its associated END keyword) is considered a single keyword instruction.

DO
  instruction
  instruction
  instruction
END

A subkeyword is a keyword that is reserved within the context of a particular instruction, for example, the symbols TO and WHILE in the DO instruction.