Currently, the ABNF requires that a collection literal used in an "in" clause requires at least one element:

inExpr = RWS "in" RWS ( listExpr / commonExpr )

listExpr = OPEN BWS primitiveLiteral BWS *( COMMA BWS primitiveLiteral BWS ) CLOSE

There may be scenarios in which the user intends to provide an empty collection (for example, to reflect that there are no items selected in a drop-down list box). This should evaluate to false.

Also, note that commonExpr may be empty, so it would seem we should have equal expressivity between commonExpr and listExpr