def context_line(lines, index, padding, prefix=nil)
      return '' if index < 1 || index > lines.size
      margin = prefix ? prefix * index.to_s.size : index.to_s
      "#{margin}#{' ' * ([padding - margin.size, 0].max)} #{lines[index - 1]}"
    end
def context_line(lines, index, padding, prefix=nil)
      return '' if index < 1 || index > lines.size
      margin = prefix ? prefix * index.to_s.size : index.to_s
      "#{margin}#{' ' * ([padding - margin.size, 0].max)} #{lines[index - 1]}"
    end
Format a single line for the error context, return empty string
 if given index is negative or greater than the lines array size

 === Parameters
 lines(Array):: Lines of text
 index(Integer):: Index of line that should be formatted for context
 padding(Integer):: Number of character to pad line with (includes prefix)
 prefix(String):: Single character string used to prefix line
                  use line number if not specified