Fitting a principal curve to a data matrix in arbitrary dimensions. 
  Hastie and Stuetzle (1989) <doi:10.2307/2289936>.

Optimization of conditional inference trees from the package 'party'
    for classification and regression.
    For optimization, the model space is searched for the best tree on the full sample by 
    means of repeated subsampling. Restrictions are allowed so that only trees are accepted 
    which do not include pre-specified uninterpretable split results (cf. Weihs & Buschfeld, 2021a).
    The function PrInDT() represents the basic resampling loop for 2-class classification (cf. Weihs 
    & Buschfeld, 2021a). The function RePrInDT() (repeated PrInDT()) allows for repeated  
    applications of PrInDT() for different percentages of the observations of the large and the 
    small classes (cf. Weihs & Buschfeld, 2021c). The function NesPrInDT() (nested PrInDT()) 
    allows for an extra layer of subsampling for a specific factor variable (cf. Weihs & Buschfeld, 
    2021b). The functions PrInDTMulev() and PrInDTMulab() deal with multilevel and multilabel 
    classification. In addition to these PrInDT() variants for classification, the function 
    PrInDTreg() has been developed for regression problems. Finally, the function PostPrInDT() 
    allows for a posterior analysis of the distribution of a specified variable in the terminal 
    nodes of a given tree.
    References are:
    -- Weihs, C., Buschfeld, S. (2021a) "Combining Prediction and Interpretation in 
    Decision Trees (PrInDT) - a Linguistic Example" <arXiv:2103.02336>;
    -- Weihs, C., Buschfeld, S. (2021b) "NesPrInDT: Nested undersampling in PrInDT" 
    <arXiv:2103.14931>;
    -- Weihs, C., Buschfeld, S. (2021c) "Repeated undersampling in PrInDT (RePrInDT): Variation 
    in undersampling and prediction, and ranking of predictors in ensembles" <arXiv:2108.05129>.

Extends the S3 generic function knit_print() in 'knitr'
    to automatically print some objects using an appropriate format such as
    Markdown or LaTeX. For example, data frames are automatically printed as
    tables, and the help() pages can also be rendered in 'knitr' documents.