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A general description is here.
‘paste()’ and ‘paste0()’ gain a new optional argument ‘recycle0’. When set to true, zero-length arguments are recycled leading to ‘character(0)’ after the ‘sep’-concatenation, i.e., to the empty string ‘""’ if ‘collapse’ is a string and to the zero-length value ‘character(0)’ when ‘collapse = NULL’.
A package whose code uses this should depend on ‘R (>= 4.0.1)’.