function GroupAddGroup takes group sourceGroup, group destGroup returns nothing
    // If the user wants the group destroyed, remember that fact and clear
    // the flag, in case it is used again in the callback.
    local boolean wantDestroy = bj_wantDestroyGroup
    set bj_wantDestroyGroup = false

    set bj_groupAddGroupDest = destGroup
    call ForGroup(sourceGroup, function GroupAddGroupEnum)

    // If the user wants the group destroyed, do so now.
    if (wantDestroy) then
        call DestroyGroup(sourceGroup)
    endif
endfunction

function GetUnitsInRectMatching takes rect r, boolexpr filter returns group
    local group g = CreateGroup()
    call GroupEnumUnitsInRect(g, r, filter)
    call DestroyBoolExpr(filter)
    return g
endfunction