  A search for non-resonant D+(s) to pi+mu+mu- and D+(s) to pi-mu+mu+ decays is
performed using proton-proton collision data, corresponding to an integrated
luminosity of 1.0 fb-1, at sqrt(s) = 7 TeV recorded by the LHCb experiment in
2011. No signals are observed and the 90% (95%) confidence level (CL) limits on
the branching fractions are
  B(D+ to pi+mu+mu-) < 7.3 (8.3) x 10-8,
  B(Ds+ to pi+mu+mu-) < 4.1 (4.8) x 10-7,
  B(D+ to pi-mu+mu+) < 2.2 (2.5) x 10-8,
  B(Ds+ to pi-mu+mu+) < 1.2 (1.4) x 10-7.
  These limits are the most stringent to date.
