title: Invalid command name in Tcl when using format
body: <p>I'm running the following commands</p>
  
  <pre><code>set full_pin_name [format "%s %s" $top_cell $encoded_name]
  puts "full_pin_name is $full_pin_name"
  </code></pre>
  
  <p>and I get the following error:</p>
  
  <pre><code>full_pin_name is invalid command name "A" B
  </code></pre>
  
  <p>when <code>top_cell</code> equals A and <code>encoded_name</code> equals B.<br>
  Why is this happening?</p>