The evaluated representation in vector form (for eval_)
with length matching the length of at, and data frame
or tibble form (for enframe_) with number of rows matching the
length of at. The at input occupies the first column,
named .arg by default, or the specification in arg_name;
the evaluated representations for each distribution in ...
go in the subsequent columns (one column per distribution). For a
single distribution, this column is named according to the
representation by default (cdf, survival, quantile, etc.),
or the value in fn_prefix. For multiple distributions, unnamed
distributions are auto-named, and columns are named
<fn_prefix><sep><distribution_name> (e.g., cdf_distribution1).