Expect semicolon after 'next' and 'last' statements.
Add 'exit' statement.
Kill reference to nonexistent operator.
Factor repetition in control statement parsing.
Factor repetition in control statements.
Add 'redo' statement.

Loop statement implementations now use 'goto' for clarity.
Improve 'next' statement description.
Document 'redo' statement.
Make 'next' statement include 'for' step.