function m = addTwo2Zero(m, from1, from2, fwd)
%ADDTWO2ZERO adds a x1 + x2 --> null reaction to the model
%
% m = addTwo2Zero(m, from1, from2, fwd) adds a reaction to the model m of
% the form:
%
%                      p(fwd)
% x(from1) + x(from2) -------> null
%

% (c) 2010 David R Hagen & Bruce Tidor
% This work is released under the MIT license.

m = a2add(m, from1, from2, from1, fwd, -1);
m = a2add(m, from1, from2, from2, fwd, -1);

m.nR = m.nR + 1;

[m.spinds.i_S, m.spinds.j_S, m.spinds.x_S, m.spinds.n_S] = ...
    sparse_vec(m.spinds.i_S, m.spinds.j_S, m.spinds.x_S, m.spinds.n_S, from1, m.nR, -1);
[m.spinds.i_S, m.spinds.j_S, m.spinds.x_S, m.spinds.n_S] = ...
    sparse_vec(m.spinds.i_S, m.spinds.j_S, m.spinds.x_S, m.spinds.n_S, from2, m.nR, -1);
