module causint {                                         # causal integration
#% _lop( xx,  yy) 
integer i, n       
real t
n = size (xx); t = 0.
if( adj){ 
                 do i= n, 1, -1 {
                   t = t + yy(i)
                   xx(i) = xx(i) + t
		}
} else {
         	do i= 1, n {
                    t   = t + xx(i)
	            yy(i) = yy(i) + t
        	}
        }
}