fubar
    while executing
"error fubar"
    ("WalkCall" body line 4)
    invoked from within
"WalkCall $avar $nvar $name $node "enter" $script"
    (procedure "::struct::tree::_walk" line 88)
    invoked from within
"::struct::tree::_walk ::mytree root {a n} {
	    if {[string equal $n "b"]} {
		lappend t .
		error fubar
	    }
	    lappend t $a $n
	}"
    ("_walk" body line 1)
    invoked from within
"mytree walk root {a n} {
	    if {[string equal $n "b"]} {
		lappend t .
		error fubar
	    }
	    lappend t $a $n
	}"
    (procedure "foo" line 7)
    invoked from within
"foo"