How to add a standard lighting model to a SCNProgram

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Standard lighting models are listed here.

let program = SCNProgram()
let programType:String = "YOUR_SHADER_PREFIX"
program.fragmentFunctionName = programType+"SurfaceFragment"
program.vertexFunctionName = programType+"VertexShader"
let shaderMaterial = SCNMaterial();
shaderMaterial.program = program;
shaderMaterial.lightingModel = .lambert;

Key addition being:

shaderMaterial.lightingModel = .lambert;

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