To provide some mild support for round-tripping variables between Stata/SPSS
and R, haven stores variable formats in an attribute: format.stata,
format.spss, or format.sas. If this causes problems for your
code, you can get rid of them with zap_formats.
See also
Other zappers:
zap_empty(),
zap_labels(),
zap_label(),
zap_widths()