A predicate may be omitted when context or a verbal cue makes it readily recoverable. Such ellipsis removes wording, not meaning: the listener supplies the missing predicate.
«فإني وقيار بها لغريب»
Indeed, both I and Qiyār are strangers here.
The predicate «لغريب» occurs once in the verse while the intended description applies to both participants. Repetition would produce a fuller literal construction but lose some of the concision and poetic binding.
Effective omission presupposes an intelligible cue. Without one, it would produce obscurity rather than eloquence.