code: def WhereIs(self, prog, path=None, pathext=None, reject=[]):
          
          if path is None:
              try:
                  path = self['ENV']['PATH']
              except KeyError:
                  pass
          elif SCons.Util.is_String(path):
              path = self.subst(path)
          if pathext is None:
              try:
                  pathext = self['ENV']['PATHEXT']
              except KeyError:
                  pass
          elif SCons.Util.is_String(pathext):
              pathext = self.subst(pathext)
          prog = SCons.Util.CLVar(self.subst(prog)) # support "program --with-args"
          path = SCons.Util.WhereIs(prog[0], path, pathext, reject)
          if path: return path
          return None
docs: Find prog in the path.