def _format_lines(lines):
    """Format the individual lines

    This will replace the '{', '}' and '~' characters of our mini
    formatting language with the proper 'where ...', 'and ...' and ' +
    ...' text, taking care of indentation along the way.

    Return a list of formatted lines.
    """
    result = lines[:1]
    stack = [0]
    stackcnt = [0]
    for line in lines[1:]:
        if line.startswith('{'):
            if stackcnt[-1]:
                s = 'and   '
            else:
                s = 'where '
            stack.append(len(result))
            stackcnt[-1] += 1
            stackcnt.append(0)
            result.append(' +' + '  ' * (len(stack) - 1) + s + line[1:])
        elif line.startswith('}'):
            stack.pop()
            stackcnt.pop()
            result[stack[-1]] += line[1:]
        else:
            assert line[0] in ['~', '>']
            stack[-1] += 1
            indent = len(stack) if line.startswith('~') else len(stack) - 1
            result.append('  ' * indent + line[1:])
    assert len(stack) == 1
    return result