title: What are forms of Ruby methods?
body: <p>We can find <a href="https://apidock.com/ruby/v1_9_3_392/Array/fill" rel="nofollow noreferrer">a definition</a> of Ruby <code>Array#fill</code> method:</p>
  
  <blockquote>
    <p>The first three forms set the selected elements of self (which may be
    the entire array) to obj. A start of nil is equivalent to zero. A
    length of nil is equivalent to self.length. The last three forms fill
    the array with the value of the block. The block is passed the
    absolute index of each element to be filled. Negative values of start
    count from the end of the array.</p>
  </blockquote>
  
  <p>What does it mean that the first three forms do something? What are these forms?</p>