def height_of_strings(strings, f, line_size):
    """
    Compute the height of a list of strings when rendered with a given font,
    taking into account the line size (i.e., the number of pixel rows
    interpolated between the bottom of one line of text and the top of the
    next).
    
    Parameters:
        strings: the list of strings whose height is measured; assumes that
            each string is a separate line.
        f: the pygame.font.Font object in which text is to be rendered.
        line_size: the number of pixel rows between lines; must be positive.
    
    Returns:
        h: the height of strings when rendered.
    """
    h = 0
    # Ignore line_size gaps to start:
    for string in strings:
        # Get current height:
        line_height = f.size(string)[1]
        h = h+line_height
    # Line gaps are added now.
    h = h+line_size*(len(strings)-1)
    return h