help of ?set_units: ... ; mention NSE


# This should not occur:
> units(b) = "10 m"
> b
Units: ^10*m
[1] 1 2 3


# This should be an error:
> b = 1:3
> units(b) = "10*m"
Warning message:
In ‘10 * m’ the numeric multiplier ‘10’ was discarded. 
The returned unit object was coerced to a value of 1.
Use `install_conversion_constant()` to define a new unit that is a multiple of another unit. 


# This should not occur:
> set_units(1:3, as_units("m"))
Units: 1
[1] 1 2 3

