"Phase Plane for X-windows." A software package that supports many common methods of nonlinear dynamical systems theory on sets of nonlinear evolution equations (ODEs, PDEs, SDEs, difference equations, delay-differential equations, integrodifferential equations). Phase plane analysis of pairs of noninear ODEs is particularly convenient. XPP was written by Bard Ermentrout of the Univ. of Pittsburgh and runs on any computing system with an X-server.