SUB span(site(,),np(),vspan,L)
    DECLARE DEF proper
    ! check for existence of vertical spanning cluster
    LET vspan = 0
    FOR bottom = 1 to L
        IF site(bottom,1) > 0 then
           LET nbot = site(bottom,1)
           FOR top = 1 to L
               IF site(top,L) > 0 then
                  LET ntop = site(top,L)
                  IF ntop = nbot then
                     LET vspan = ntop
                     EXIT SUB
                  END IF
               END IF
           NEXT top
        END IF
    NEXT bottom
END SUB