Lower type bound ($U<:V$)

A lower bound requires an admissible type argument to be a supertype of a specified type. In Scala, [V >: U] means $U<:V$. For an immutable software class covariant in U, a method accepting a new output value cannot consume U directly in a contravariant method-parameter position. Instead, add[V >: U](x: V) accepts a possibly wider value and returns a fresh instance parameterized by V. The original narrower instance remains unchanged. The variance-position rule and subtyping justify this widening; a bound alone does not authorize mutating an existing instance through a widened reference.^[ref]