vec_arith for iso_double_with_units

# S3 method for iso_double_with_units
vec_arith(op, x, y, ...)

## Arguments

op An arithmetic operator as a string A pair of vectors. For !, unary + and unary -, y will be a sentinel object of class MISSING, as created by MISSING(). A pair of vectors. For !, unary + and unary -, y will be a sentinel object of class MISSING, as created by MISSING(). These dots are for future extensions and must be empty.