Syntax:  SET PLACEHOLDER INHERIT placeholder_name
              defined_placeholder defined_language

  Causes a placeholder to inherit the definition of another
  placeholder.

  The placeholder specified for the defined_placeholder parameter will
  inherit the definition of the placeholder specified for the
  placeholder_name parameter.  This command is valid only on alias-type
  placeholders.  Since placeholders of this type inherit all their
  attributes from the specified defined placeholder, no other
  attributes can be set on them.  A placeholder defined with this
  command may not be used to define any other placeholder.