[KBK] closed a follow-up to [the comp.lang.tcl newsgroup] with the
advice that,
"Also, it's important always, always to brace your [expr]essions.
Don't do

     set a [expr $b / $c]

but rather

     set a [expr { $b / $c }]

Otherwise, your variables may undergo unexpected conversions
numeric -> string -> numeric, you can lose precision, your
expressions will be much slower, and you can even have security
problems:

     # don't run
     set x { [format C:\] }
     set y [expr $x + 3]
"
----
[RS]: In the following cases braced expressions don't work:
   * operator in variable (which in most languages is impossible, but Tcl can do):
 set op +
 expr $x $op $y
   * (parts of) expression generated by string manipulation:
 expr [join $intlist +]

----
[KPV]: Braced expressions also don't work when you do
negation in the lazy way:
  set n1 -3
  set n2 -$n1
  expr {2 + $n2} ;# error
  expr 2 + $n2   ;# ok