measure fit: selections must have the same number of atoms
measure fit: selections must have the same number of atoms
    while executing
"measure fit $sel1 $sel2"
    (procedure "align" line 5)
    invoked from within
"align $sel($i) $sel($reference_for_alignment)"
    (procedure "align_all" line 18)
    invoked from within
"align_all $arg1 $rmsd_base"
    (in namespace inscope "::RMSDTool" script line 11)
    invoked from within
"::namespace inscope ::RMSDTool {
    if {$rmsd_base=="ave"} {
      tk_messageBox -type ok -title "RMSD Error" -message "You must choose either Top or..."
    invoked from within
".rmsdtool.top.right.align invoke"
    ("uplevel" body line 1)
    invoked from within
"uplevel #0 [list $w invoke]"
    (procedure "tk::ButtonUp" line 23)
    invoked from within
"tk::ButtonUp .rmsdtool.top.right.align"
    (command bound to event)