docs: def format(self, fmt, **kwargs):
          """
          Hooks compute to generate a value from a format string.
          """
  
          def compute(self):
              values = {}
              try:
                  for name, field in kwargs.iteritems():
                      values[name] = reduce(getattr, field.split('.'), self.ctx.form)
              except AttributeError, ex:
                  self.ctx.errors.invalid(str(ex))
                  return ERROR
              return fmt.format(**values)
  
          return self.compute.attach(self)(compute)