Remove an extra "for"
Update Documenting-functions.rmd

Removed a line about the `@genericMethods` tag, which is not implemented in roxygen2.
sorry if the omission was intentional
case sensitive
Fixed typo
Update Functionals.rmd
fixed typo at line 730, from 'solve' to 'solving'
Update Data-structures.rmd

Fixed typo.
Fix type name
Copy-editing for chapter 1
Add index
Copy-editing for chapter 2
Add index for chapter 2.
Plus lots of stuff to make it more robust
Add description of SEXPS
Copy-editing for chapter 3
Indexing chapter 3
Minor index tweaks
More minor index tweaks
Update Data-structures.rmd

typo
May not return value in this else-if branch

test case:

```rconsole
> logical_abbr(quote(function(x=TRUE, na.rm = TRUE) FALSE ))
Error in if (logical_abbr(x[[i]])) return(TRUE) : 
  argument is of length zero
```