#' Union of BiocParallelParam and missing
#' 
#' Allows a method parameter to be either a BiocParallelParam class object or
#' missing.
#' 
#' 
#' @name BiocParallelParamOrMissing
#' @aliases BiocParallelParamOrMissing BiocParallelParamOrMissing-class
#' @docType class
#' @author Dario Strbenac
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#' Union of character and missing
#' 
#' Allows a method parameter to be either a character or missing.
#' 
#' 
#' @name characterOrMissing
#' @aliases characterOrMissing characterOrMissing-class
#' @docType class
#' @author Dario Strbenac
#' @examples
#' 
#'   ModellingParams("upsample")
#'   ModellingParams() # Default is "downsample" if it is missing.
#' 
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#' Union of A DataFrame Object and NULL
#' 
#' Allows a slot to be either a DataFrame class object or empty. No
#' constructor.
#' 
#' 
#' @name DataFrameOrNULL
#' @aliases DataFrameOrNULL DataFrameOrNULL-class
#' @docType class
#' @author Dario Strbenac
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#' Union of numeric and missing
#' 
#' Allows a method parameter to be either a numeric value or missing.
#' 
#' 
#' @name numericOrMissing
#' @aliases numericOrMissing numericOrMissing-class
#' @docType class
#' @author Dario Strbenac
#' @examples
#' 
#'   CrossValParams(permutations = 10) # numeric
#'   CrossValParams() # missing
#' 
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