A function to be used as a symbolic description of some fitting models such as svm. This function instructs the model to do a linear transformation of the input values. The `predictors_index` parameter informs the positions of `tb` fields corresponding to formula independent variables. If no value is given, the default is NULL, a value indicating that all fields will be used as predictors.
sits_formula_linear(predictors_index = -2:0)
Index of the valid columns whose names are used to compose formula (default: NULL).
A function that computes a valid formula.