function [y,n]=sigadd(x1,n1,x2,n2)
n=min(min(n1),min(n2)):max(max(n1),max(n2))
y1=zeros(1,length(n));y2=y1;
y1(find((n>=min(n1))&(n<=max(n1))==1))=x1
y2(find((n>=min(n2))&(n<=max(n2))==1))=x2
y=y1+y2;