Returns the COS code to compute this expression. Sets pInline to 1 if the returned string can simply be nested in an enclosing expression. If pInline = 0, the code returned is assumed to populate a variable tValue, with each line prepended with appropriate whitespace to allow simple compilation.
If the expression can evaluate to anything other than valid values, an additional variable tStatus should be populated with the appropriate status (which will likely result in pInline being 0)
If pOverrideSource is set, it should be used rather than the expected "input" for this expression (ie: ..field)