def get_val x, y
    if !x.is_a?Integer
      Error.call "Matrix::get_val : '#{x}' is not a correct line"
      return nil
    elsif !y.is_a?Integer
      Error.call "Matrix::get_val : '#{y}' is not a correct column"
      return nil
    elsif x < 0 or y < 0 or x >= @lines or y >= @columns
      Error.call "Matrix::get_val : The specified positions are invalids (#{x},#{y})"
      return nil
    else
      return @v[x][y]
    end
  end
def get_val x, y
    if !x.is_a?Integer
      Error.call "Matrix::get_val : '#{x}' is not a correct line"
      return nil
    elsif !y.is_a?Integer
      Error.call "Matrix::get_val : '#{y}' is not a correct column"
      return nil
    elsif x < 0 or y < 0 or x >= @lines or y >= @columns
      Error.call "Matrix::get_val : The specified positions are invalids (#{x},#{y})"
      return nil
    else
      return @v[x][y]
    end
  end
== Parameters:
 x,y::
   Integers. They are the coordonates of the value which will extract from the matrix
 == Returns:
 a value fo the matrix