title: In TCL, what what does if { [a 0] } mean? 'a' is defined true
body: <p>I am referring tcl code of my project, I don't understand what it means. </p>
  
  <p>TCL Usage: </p>
  
  <pre><code>if {[Variable 0] } {
    return 1
  }
  </code></pre>
  
  <p>Please help! </p>