SUBROUTINE wextend(n,x,lf,y)
    implicit none
    
    integer :: n,lf
    real*8 :: x(n),y(n+2*lf)
    
    integer :: i
    
    y = 0
    
    if(n>lf) then
        do i=1,lf
            y(i) = x(lf-i+1)
        end do
        do i=lf+1,lf+n
            y(i) = x(i-lf)
        end do
        do i=lf+n+1,n+2*lf
            y(i) = x(lf+2*n+1-i)
        end do
    else
        goto 10
    end if
    
10  end subroutine