Documentation for concat can be found at
http://purl.org/tcl/home/man/tcl8.4/TclCmd/concat.htm

When concat puts its arguments together, it seperates them
with a space.

----

To concat an explicit empty list and another list use:

  set a [concat {{}} {a} {b}]

'''[DGP]''' No, that example concatenates three single-element
lists into a single three-element list.  Likewise, so does

  set a [concat [list {}] a b]

Don't confuse an empty list (a list of 0 elements), with a 
single-element list whose element happens to be an empty string.

Also don't imagine that braces have magic list-ifying powers.

----

When does one use concat compared to other Tcl methods of putting two strings
together?

See [Concatenating lists] for a timing comparison of various methods.

Other methods of putting strings together include:

   * string substitution
       set a "abc"
       set b "123"
       set c $a$b

   * [format] command
       set c [format "%s %s" $a $b]

----
Concatenating the sublists of a list (e.g. a matrix) is best done with [join].
----
[[
[Tcl syntax help]
- [Arts and crafts of Tcl-Tk programming]
- [Category Command] of [Tcl]
]]