'''[dodekalogue%|%Command Substitution]''' is one of the fundamental features
of Tcl, in which a [script] is evaluated and the result is substituted into a
word in a [command].  A new stack frame is not created when the script is
evaluated, so using `[return]` or `[break]` or the like will cause the caller
to return, etc. 



** Recursion **

[PYK] 2019-02-12 2019-11-24:  To perform each command subsitution, including
nested substitutions, Tcl evaluates the script at the current level. No
additional [level%|%levels] are created:

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



** Comments **

[AMG]: Command substitution can be used in combination with [list] and [{*}] to embed comments in lists.  The syntax is awkward, but it does work:

======
switch -regexp $input [list {*}[
# Handle words starting with a capital letter
] {^[A-Z]} {
    # Do the thing
    theThing
} {*}[
# Handle everything else
] default {
    # Don't do the thing
}]
======

Though also consider using [decomment] for this same purpose.

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