def library_source(lines):
    """
    Return source code for a VisionCpp library.

    The generated code will compile a callable library method
    "natvie_expression_tree()".

    Arguments:
        lines (str[]): Program implementation.

    Returns:
        str: Source code
    """
    program_lines = ['#include <opencv2/opencv.hpp>',
        '#include <visioncpp.hpp>', '', 'extern "C" {', '',
        'int native_expression_tree() {'] + lines + ['}', '',
        '}  // extern "C"']
    return '\n'.join(program_lines) + '\n'
