TODO

This text is not a task.

Bugs

revisit some of the check_ functions. I think the default approach should be the aim of the check_() functions is only to verify the input is as expected. Currently these functions do a lot more:
- check_col_plan_dots()
- check_span_structure_dots()
revisit is_valid_() functionality
- the constructor should always produce a valid object
functions should check their inputs. If the function is not user facing, then it should report the error / condition from the calling function. For example, apply_tfrmt() should have a call = rlang::caller_env() argument.
all user-facing functions should check all their inputs before doing any work.
- for example, big_n_structure() should check param_val
all S3 classes introduced by {tfrmt} should have matching is_<class>() and check_<class>() functions.

prefer expect_equal() to expect_identical() (contrary to {lintr}). Follow Hadley's advice: "Numerical precision can also vary across platforms, so use expect_equal() unless you have a specific reason for using expect_identical()." @dragosmg