function [pos, E] = bp_chain(n, len)
% BP_CHAIN A straight line of n beads along x, from 0 to len.
%   [pos, E] = bp_chain(n, len) returns positions (n-by-3) and the n-1
%   neighbor pairs E. The end beads are 1 and n.
x = linspace(0, len, n)';
pos = [x, zeros(n, 2)];
E = [(1:n-1)', (2:n)'];
end