safe.mapping: Maps with a left_join, ensuring constant number of rows
In xrobin/xavamess: Useful Miscellaneous Functions

This avoids generating left duplicates if entries are duplicated in the right by. If duplicates are generated, the function will stop with an error.

1	safe.mapping(x, map, by = NULL)

`x`	the `data.frame` with fixed size
`map`	a `data.frame` with the new columns.
`by`	see `left_join`

a <- data.frame(A = LETTERS[1:5], B = 1:5)
b <- data.frame(A = LETTERS[1:5], C = 15:11)
dplyr::left_join(a, b)
safe.mapping(a, b)

# Now with duplicates
## Not run: 
c <- data.frame(A = LETTERS[1:5], C = 20:11)
dplyr::left_join(a, c) # creates duplicates of x
safe.mapping(a, c) # stops with an error

## End(Not run)