code: def which(self, cmd, parent_environ=None, fallback=False):
          
          env = self.get_environ(parent_environ=parent_environ)
          path = which(cmd, env=env)
          if fallback and path is None:
              path = which(cmd)
          return path
docs: Find a program in the resolved environment.
  
          Args:
              cmd: String name of the program to find.
              parent_environ: Environment to interpret the context within,
                  defaults to os.environ if None.
              fallback: If True, and the program is not found in the context,
                  the current environment will then be searched.
  
          Returns:
              Path to the program, or None if the program was not found.