These functions provide tidy eval-compatible ways to capture
symbols (`sym()`, `syms()`, `ensym()`), expressions (`expr()`,
`exprs()`, `enexpr()`), and quosures (`quo()`, `quos()`, `enquo()`).
To learn more about tidy eval and how to use these tools, read
<http://rlang.tidyverse.org/articles/tidy-evaluation.html>