chan push


    namespace eval tee {
        proc initialize args {info procs}
        proc finalize args {}
        proc clear args {}
        proc flush {x h} {chan flush $x}
        proc write {x h data} {
            puts -nonewline $x $data
            chan flush $x
            return $data
        }
        namespace export *
        namespace ensemble create -parameters x
    }
    namespace eval redir {
        proc initialize args {info procs}
        proc finalize args {}
        proc clear args {}
        proc flush {x h} {chan flush $x}
        proc write {x h data} {
            uplevel 1 $x [list $data]
            return ""
        }
        namespace export *
        namespace ensemble create -parameters x
    }


    package require Tk
    pack [text .t]
    set fd [open log.txt w]
    chan push stdout [list tee $fd]
    chan push stderr [list redir {.t insert end}]

    puts "Hello world!"
    puts stderr "This is not right"