the set of points in the convex hull
let a = [ ] a . push ( [ 0 , 0 ] ) ; a . push ( [ 3 , - 1 ] ) ; a . push ( [ 4 , 5 ] ) ; a . push ( [ - 1 , 4 ] ) ; let n = a . length ; let p = [ 100 , 100 ] ; a = addPoint ( a , p ) ;