/usr/share/tcltk/tcllib1.16/control/assert.tcl is in tcllib 1.16-dfsg-2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 | # assert.tcl --
#
#	The [assert] command of the package "control".
#
# RCS: @(#) $Id: assert.tcl,v 1.3 2004/01/15 06:36:12 andreas_kupries Exp $
namespace eval ::control {
    namespace eval assert {
	namespace export EnabledAssert DisabledAssert
	variable CallbackCmd [list return -code error]
	namespace import [namespace parent]::no-op
	rename no-op DisabledAssert
	proc EnabledAssert {expr args} {
	    variable CallbackCmd
	    set code [catch {uplevel 1 [list expr $expr]} res]
	    if {$code} {
		return -code $code $res
	    }
	    if {![string is boolean -strict $res]} {
		return -code error "invalid boolean expression: $expr"
	    }
	    if {$res} {return}
	    if {[llength $args]} {
		set msg [join $args]
	    } else {
		set msg "assertion failed: $expr"
	    }
	    # Might want to catch this
	    namespace eval :: $CallbackCmd [list $msg]
	}
	proc enabled {args} {
	    set n [llength $args]
	    if {$n > 1} {
		return -code error "wrong # args: should be\
			\"[lindex [info level 0] 0] ?boolean?\""
	    }
	    if {$n} {
		set val [lindex $args 0]
		if {![string is boolean -strict $val]} {
		    return -code error "invalid boolean value: $val"
		}
		if {$val} {
		    [namespace parent]::AssertSwitch Disabled Enabled
		} else {
		    [namespace parent]::AssertSwitch Enabled Disabled
		}
	    } else {
		return [string equal [namespace origin EnabledAssert] \
			[namespace origin [namespace parent]::assert]]
	    }
	    return ""
	}
	proc callback {args} {
	    set n [llength $args]
	    if {$n > 1} {
		return -code error "wrong # args: should be\
			\"[lindex [info level 0] 0] ?command?\""
	    }
	    if {$n} {
	        return [variable CallbackCmd [lindex $args 0]]
	    }
	    variable CallbackCmd
	    return $CallbackCmd
	}
    }
    proc AssertSwitch {old new} {
	if {[string equal [namespace origin assert] \
		[namespace origin assert::${new}Assert]]} {return}
	rename assert ${old}Assert
	rename ${new}Assert assert
    }
    namespace import assert::DisabledAssert assert::EnabledAssert
    # For indexer
    proc assert args #
    rename assert {}
    # Initial default: disabled asserts
    rename DisabledAssert assert
}
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