This command joins each of its arguments together with spaces after trimming leading and trailing white-space from each of them. If all the arguments are lists, this has the same effect as concatenating them into a single list. It permits any number of arguments; if no args are supplied, the result is an empty string.
concat a b {c d e} {f {g h}}
will return "a b c d e f {g h}" as its result), it will also concatenate
things that are not lists, and hence the command: concat " a b {c " d " e} f"
will return "a b {c d e} f" as its result.
Note that the concatenation does
not remove spaces from the middle of its arguments, so the command:
concat "a b c" { d e f }
will return "a b c d e f" (i.e. with three spaces between the a, the
b and the c).