docs: function (c) {
  
  // If a c parameter is provided, verify that it matches the current character.
  
              if (c && c !== ch) {
                  error("Expected '" + c + "' instead of '" + ch + "'");
              }
  
  // Get the next character. When there are no more characters,
  // return the empty string.
  
              ch = text.charAt(at);
              at += 1;
              return ch;
          }