text: In fluid dynamics, two types of stream function are defined:
  * The two-dimensional (or Lagrange) stream function, introduced by Joseph Louis Lagrange in 1781, is defined for incompressible (divergence-free), two-dimensional flows.
  * The Stokes stream function, named after George Gabriel Stokes, is defined for incompressible, three-dimensional flows with axisymmetry.
  The properties of stream functions make them useful for analyzing and graphically illustrating flows.
  The remainder of this article describes the two-dimensional stream function.