Function tangent_angle 

pub fn tangent_angle(
    curve: &PlanarCurve,
    t: f64,
    tol: Tolerances,
) -> OgeomResult<f64>

The angle a planar curve’s tangent makes with the u axis at t.

The natural way to ask which way a pcurve is heading, which is what wire ordering and outer/inner classification are built from.

Errors

OgeomError::Construction at a cusp, where the derivative vanishes.