def _conditional_field_block(if_keyword, expression, colon, comment, newline,
                             indent, fields, dedent):
  """Applies an existence_condition to each element of fields."""
  del if_keyword, newline, colon, comment, indent, dedent  # Unused.
  for field in fields.list:
    condition = field.field.existence_condition
    condition.CopyFrom(expression)
    condition.source_location.is_disjoint_from_parent = True
  return fields