iRule to drop HTTP GET requests that don't have zero content length header

That's a good question....is "not exist" a valid qualifier? so...would this work?

when HTTP_REQUEST { if { ( [HTTP::method] equals "GET" ) and ((( [HTTP::header exists Content-Length] ) and ( [HTTP::header Content-Length] == 0 )) or ( [HTTP::header not exist Content-Length] )) } { do nothing } else { reject } }