# The tilda prefixed parameter in the Spec

From the spec:

and:

StatementList : ReturnStatement ExpressionStatement[~In]

is equivilant to:

StatementList : ReturnStatement ExpressionStatement

What is the point of the tilda prefixed parameter appended to non-terminals if there is no effect in the result?

You have to understand that equivalence in conjunction with the expansion for the + prefix for grammatical parameters. E.g.,

`Foo[+In]` expands to `Foo_In`, and
`Foo[~In]` expands to `Foo`.

What's important is that [+In] and [~In] have different expansions. The particular expansions shown aren't that important; they're never used elsewhere in the spec. The syntax would be the same if the spec said:

`Foo[+In]` expands to `Foo_plusIn`, and
`Foo[~In]` expands to `Foo_tildeIn`.

or

`Foo[+In]` expands to `Foo+In`, and
`Foo[~In]` expands to `Foo~In`

(if we assume that the world of expanded nonterminals allows + and ~ in non-terminal names).

Personally, I think a more symmetric expansion (like the alternatives above) would be less weird for the reader.