Function compose_derived 

pub fn compose_derived(elements: &[(String, i32)]) -> String

Compose a derived-unit display symbol from (element_symbol, exponent) pairs, e.g. [("m", 3), ("s", -1)] -> m\u{00B3}/s, [("J",1),("kg",-1),("K",-1)] -> J/(kg\u{00B7}K). Elements with exponent 0 are dropped. Returns an empty string only when there are no non-zero elements.