function new_map=contract_mappings(mapping_1,mapping_2)
% Constructs a new mapping by combining two earlier mappings. Use of this
% function in series may help combine any number of mappings.
%
%mapping_1 - (number of particles)x2 column matrix
%
%For bugs, comments, etc. please contact Kapil Krishan :
%kkrishan@uci.edu
%www.physics.uci.edu/~foams
%version: I

[C,IA,IB]=intersect(mapping_1(:,2),mapping_2(:,1));
new_map=[mapping_1(IA,1) mapping_2(IB,2)];