Insert an element in the container.
If allowDuplicates is false, this can fail and return false
if the already contains an element with equivalent key.
structSetImpl(K, alias less = (a, b) => a < b, bool allowDuplicates = false, bool ownsMemory = false)
Insert an element in the container. If allowDuplicates is false, this can fail and return false if the already contains an element with equivalent key.