Documentation for concat can be found at http://purl.org/tcl/home/man/tcl8.4/TclCmd/concat.htm
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.
Concatenating the sublists of a list (e.g. a matrix) is best done with join.
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