Function shape_function_gradient_ty 

pub fn shape_function_gradient_ty() -> Expr

ShapeFunctionGradient : ReferenceTriangle → Nat → (Real → Real → Real × Real)

Gradient of the k-th shape function on the reference element. For P1: ∇φ₁ = (-1,-1), ∇φ₂ = (1,0), ∇φ₃ = (0,1).