TCL : subst command should do command , variable , backslash substitution

TCL : subst command should do command , variable , backslash substitution

TCL : subst command should do command , variable , backslash substitution .
proc sum {a b} {
return [expr $a+$b]
}
%
% set a 1
1
% set b 2
2
subst {sum {$a} {$b}} >>>>>>>>>>>>>>> subst command should do the variable
and command subsdtitutions , Here why command substitution not happend .
sum {1} {2}
ubst command should do the variable and command subsdtitutions , Here why
command substitution not happend . why it is only substituting the
variables ?