opengl - vertex index in SceneKit shader modifier? -

i'm trying implement parametric surface in scenekit using shader modifier in form of vertex shader program in glsl.

the problem there not seem input variable vertex index among inputs provided scenekit. in opengl, vertex index provided gl_vertexid input.

the documentation pretty sparse input , output variables in shader modifiers, i'm hoping can confirm vertex index not available. in case, i'd appreciate suggestions getting around in context of vertex shader program in scenekit.