subroutine boxconv( nb, nx, xx, yy)
# inputs:	nx,  xx(i), i=1,nx	the data
#		nb			the box length
# output:	yy(i),i=1,nx+nb-1	smoothed data
integer nx, ny, nb, i
real xx(nx), yy(1)
temporary real bb(nx+nb)
if( nb < 1 || nb > nx)	call erexit('boxconv')	# "||" means .OR.
ny = nx+nb-1
do i= 1, ny
	bb(i) = 0.
bb(1) = xx(1)
do i= 2, nx
		bb(i) = bb(i-1) + xx(i)		# make B(Z) = X(Z)/(1-Z)
do i= nx+1, ny
		bb(i) = bb(i-1)
do i= 1, nb
		yy(i) = bb(i)
do i= nb+1, ny
		yy(i) = bb(i) - bb(i-nb)	# make Y(Z) = B(Z)*(1-Z**nb)
do i= 1, ny
		yy(i) = yy(i) / nb
return; end