def at_depth(depth , *strings)
      prefix = " " * 2 * depth
      strings.collect{|str| prefix + str}.join("\n")
    end
def at_depth(depth , *strings)
      prefix = " " * 2 * depth
      strings.collect{|str| prefix + str}.join("\n")
    end
helper method for formatting source code
 depth is the depth in the tree (os the ast)
 and the string are the ones to be indented (2 spaces)