title: Why the subst command only does variable substitution in my case?
body: <p>Tcl's <code>subst</code> command should do command , variable , backslash substitution .</p>
  
  <pre><code>proc sum {a b} {
      return [expr $a+$b]
  }
  
  %
  % set a 1
  1
  % set b 2
  2
  
  subst {sum {$a} {$b}}
  </code></pre>
  
  <p><code>subst</code> command should do the variable and command subsdtitutions , Here why command substitution not happend  .</p>
  
  <pre><code>sum {1} {2}
  </code></pre>
  
  <p><code>subst</code> command should do the variable and command subsdtitutions , Here why command substitution not happend  . why it is only substituting the variables ?</p>