[ocl25] Implement ExpCS.getDerivedArgument()
[unrelated] Avoid no-manifest problem
[ocl25] Implement ExpCS.getDerivedSource()
[ocl25] Implement ExpCS.getDerivedParent()
[ocl25] Determine logical expression tree with derived properties
[ocl25] Rationalize InfixExpCS/PrefixExpCS properties
[ocl25] LetVariableCS is ExpCS first, VariableCS second
tidy up