A multiple-selection switch is found in many languages, in slightly differing forms. The Switch Statement of C# and Java has syntax that can be described by

but, as usual, this has to be supplemented by semantic constraints. Among these are, fairly obviously, that no Constant may appear more than once as a Case Label within a single Switch Statement, that the types of all the constants and the type of the selector Expression must be compatible, and that there may be only one default label. Although not illegal, a Switch Statement without any instances of One Switch is of little use and might elicit a warning to the user if it is detected. In C, C++ and Java, the statements that follow each Case Label are not restricted in any way, although it is common to find that the last of them is a Break Statement or Return Statement – if absent the semantics prescribe that control then falls through to the next option. This is expressly forbidden in C# which requires that if a Statement Sequence is not empty (or is comprised only of empty statements) then it must not be possible to fall through to the next option. To a good approximation, this means that each Statement Sequence must terminate with an unguarded explicit Break Statement or Return Statement. Develop the productions given above in such a way that all these constraints are properly checked.