“A proviso is sometimes added to allay fears which may be unfounded with the result that a statute is burdened with a needless proviso; for if the enacting clause does not have the effect which the proviso seeks to qualify, there is no point in adding the proviso. The trouble arises from assuming that the enactment has the effect implied by the proviso; one then goes on to infer that it does really have that effect, and to import legislation into the body of the statute; so that a proviso intended to cut down or qualify the enacting clause has the opposite result and enhances the effect of the clause.”