A secure contract may not be varied except—
in accordance with sections 104 to 107, or
by or as a result of an enactment.
A variation of a secure contract (other than by or as a result of any enactment) must be in accordance with section 108.
This section is a fundamental provision which is incorporated as a term of all secure contracts; section 20 provides that subsections (1)(b) and (2) of this section—
must be incorporated, and
must not be incorporated with modifications.