Test whether each term is a scalar.
Value
A logical vector indicating whether the term is a scalar.
Examples
scalar_term(term("alpha[1]", "alpha[3]", "beta[1]", "sigma[3]"))
#> [1] FALSE FALSE TRUE FALSE
scalar_term(term("alpha[1]", NA_term_, "beta[1]", "beta[3]"))
#> [1] TRUE NA FALSE FALSE