Create new modifiables using a structure creation function.
Remove commented-out code.
new_modifiable is an IOAction1, not an IOAction3!
A new modifiable must be marked changeable.
Catch changeable let expressions early.
Allow OperationArgs to specify changeability, but default to that of parent.
Make perform_expression( ) default to NOT evaluating changeables.
Discover modifiables without evaluating changeables.
Add function parameter_constructor( ).
Rewrite framework: pass down ec and make creation of new modifiables work.
Fix new_modifiable_list.
Fix creating parameters based on their distribution.