docs: def do_schema(self, line):
          """
          schema
  
          schema --clear
  
          schema filed_name field_type
          """
          args = self.getargs(line)
  
          if not args:
              for k in sorted(self.schema):
                  print "%s\t%s" % (k, self.schema[k])
          elif args[0] == "--clear":
              self.schema.clear()
          else:
              fname = args[0]
              ftype = args[1]
  
              if not ftype in VALID_TYPES:
                  print "invalid type: use %s" % list(VALID_TYPES)
              else:
                  self.schema[fname] = ftype