Provides computational resources for nonlinear longitudinal models, specifically those that are intrinsically nonlinear, in four distinct contexts: (1) univariate longitudinal processes represented by latent variables, with an option of incorporating covariates such as time-invariant covariates (TICs) and time-varying covariates (TVCs); (2) multivariate longitudinal sequences that facilitate the assessment of correlations or causative relationships between longitudinal variables; (3) multiple-group structures for models found in scenarios (1) and (2), allowing for the exploration of disparities among manifested classes; and (4) mixture models for scenarios (1) and (2), premised on the assumption that trajectories are from multiple latent classes. The methods implemented are based on Peugh & Fan (2015) <doi:10.1080/10705511.2014.919823>, Lubke & Muthén (2007) <doi:10.1080/10705510709336735>, Casella & Berger (2002, ISBN: 9780534243128), Madansky (1965) <https://www.jstor.org/stable/1266390>, Matthews (1988) <https://www.jstor.org/stable/2336444>, Efron & Tibshirani (1994, ISBN: 9780412042317>, Estabrook & Neale (2013) <doi:10.1080/00273171.2012.730072>, Priestley & Subba Rao (1975) <doi:10.1080/00207177508922050>, Dumenci (2011) <doi:10.1177/1094428110374649>, Landis & Koch (1977) <doi:10.2307/2529310>, Agresti (2012, ISBN: 9780470463635), Liu & Perera (2023) <arXiv:2201.00470v6>, Liu (2022) <arXiv:2210.16916>, Grimm, Zhang, Hamagami & Mazzocco (2013) <doi:10.1080/00273171.2012.755111>, Liu, Perera, Kang, Sabo, and Kirkpatrick (2021) <doi:10.3102/10769986211052009>, Sterba (2014) <doi:10.1080/10705511.2014.919828>, Liu & Perera (2021) <doi:10.1037/met0000309>, Blozis (2004) <doi:10.1037/1082-989X.9.3.334>, Liu & Perera (2022) <doi:10.3758/s13428-022-01940-2>, MacKinnon (2008, ISBN: 9780805864298), Cheong, MacKinnon & Khoo (2003) <doi:10.1207/S15328007SEM1002_5>, Soest & Hagtvet (2011) <doi:10.1080/10705511.2011.557344>, Liu & Perera (2022) <doi:10.1037/met0000436>, Liu & Perera (2022) <doi:10.1037/met0000500>, and Liu (2023) <arXiv:2301.06014>.