The nonlinear pendulum with no damping, 
no driving. Change initial speed and see 
what happnes as amplitude increases.

initial speed

initial position

b = 1.5 (slider from -6 to 6, step 0.3)
gr: 9.8
l: 2
y2'(t, y1, y2) = (((-gr)) / l * sin(y1))
y1'(t, y1, y2) = y2
a = 0 (slider from -10 to 10, step 1)
angle, speed = NSolveODE({y1', y2'}, 0, {a, b}, 20)
c = 0.61 (slider from 0 to 1, step 0.002)
x1: (l * sin(y(Point[angle, c])))
z1: ((-l) * cos(y(Point[angle, c])))
pendulum: ((2 * x1) - 6, (2 * z1) + 0)
f(y1, y2) = y2
g(y1, y2) = (((-gr)) / l * sin(y1))
B: (a, b)
phaseplot = SolveODE(g, f, x(B), y(B), 10, 0.001)
pendulum_1 = Segment((-6, 0), pendulum)
bd: 1.4
ln_1: 0.3
len: 1.1