code: def Requires(self, target, prerequisite):
          
          tlist = self.arg2nodes(target, self.fs.Entry)
          plist = self.arg2nodes(prerequisite, self.fs.Entry)
          for t in tlist:
              t.add_prerequisite(plist)
          return tlist
docs: Specify that 'prerequisite' must be built before 'target',
          (but 'target' does not actually depend on 'prerequisite'
          and need not be rebuilt if it changes).