double precision function exa(x,y,t,ax,ay)

c General information:

c     Function exa returns an exact solution to the two-dimensional
c     convection equation
c
c        u_t + ax u_x + ay u_y =0.

c     Function exa requires an arbitrary two parameter function
c        f(x,y).
c     The returned exact solution will be
c        f(x - ax t, y - ay t).

c     Copyright 1997 Leon van Dommelen
c     Version 1.0 Leon van Dommelen 1/10/97

c Arguments:

c     Avoid typos:
      implicit none

c     Input: the position for which to return the exact solution:
      double precision x,y

c     Input: the time at which to return the exact solution:
      double precision t

c     Input: the components ax and ay of the convection velocity:
      double precision ax,ay

c External variables:

c     An arbitrary two-parameter function:
      double precision f
      external f

c Executable statements:

c     Return the exact solution:
      exa=f(x-ax*t,y-ay*t)

c Exit:

      return
      end