'''[Script] Substitution''' is the more apt term for [Dodekalogue%|%Command
Substitution], since entire scripts, not just individual commands, can be
contained in brackets.  The result of the script is the result of the final
command. A new stack frame is not created, so using `[return]` or `[break]`
or the like will cause the caller to return, etc. 


** Recursion **

[PYK] 2019-02-12:  Script substitution can be recursive.  Everything happens at
the same level, as no additional [level%|%levles] are created:

======
puts [
    set n 10
    set a {
        puts $n
        expr {[incr n -1]? [try $a] : {liftoff}}
    }
    try $a
]
======


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