Semi-Parametric Shape-Restricted Fixed/Mixed Effect(s) Regression Spline // Select the most suitable shape to describe the relationship between the exposure and the outcome among increasing, decreasing, convex, and concave shapes (Yin et al. (2021) DOI:10.1007/s13571-020-00246-7); estimate the direct and indirect effects with prior knowledge on the relationship between the mediator and the outcome with binary exposure (Yin et al. (2024) DOI:10.1007/s13571-024-00336-w); estimate the direct and indirect effects using linear regression-based approach (VanderWeele (2015, ISBN:9780199325870)).