add

$group add element: intakes element

obj method group add args {
  foreach element $args {
    if {$element eq $self} then {
      return -code error "cannot add $self to group $self"
    }
    if {$element ni [my elements]} then {
      my elements [list {*}[my elements] $element]
      $element group $self
    }
  }
}