연습문제 2.2

function make_point(x, y){
    return pair(x, y);
}

function x_point(p){
    return head(p);
}

function y_point(p){
    return tail(p);
}

function make_segment(start, end) {
    return pair(start, end)
}

function start_segment(s){
    return head(s);
}

function end_segment(s){
    return tail(s);
}

function midpoint_segment(s){
    return make_point( x_point(start_segment(s))+x_point(end_segment(s))/2, 
    y_point(start_segment(s))+y_point(end_segment(s)/2))
}