This command performs variable substitutions, command substitutions, and
backslash substitutions on its _s_t_r_i_n_g argument and returns the fully-
substituted result. The substitutions are performed in exactly the same
way as for Tcl commands. As a result, the _s_t_r_i_n_g argument is actually
substituted twice, once by the Tcl parser in the usual fashion for Tcl
commands, and again by the _s_u_b_s_t command.
If any of the ----nnnnoooobbbbaaaacccckkkkssssllllaaaasssshhhheeeessss, ----nnnnooooccccoooommmmmmmmaaaannnnddddssss, or ----nnnnoooovvvvaaaarrrriiiiaaaabbbblllleeeessss are specified,
then the corresponding substitutions are not performed. For example, if
----nnnnooooccccoooommmmmmmmaaaannnnddddssss is specified, no command substitution is performed: open and
close brackets are treated as ordinary characters with no special
interpretation.
Note: when it performs its substitutions, _s_u_b_s_t does not give any special
treatment to double quotes or curly braces. For example, the script