Why are some verbs, such as "immineo, -, -, ere" listed in the Latin dictionary

with only two principle parts? Presumably, the parts omitted don't exist - but then I'm curious as to why this is. Could anyone explain why certain verbs are listed without the first person perfect or perfect passive participle, while most others are. (Immineo, in this dictionary has the meanings "to project over", "lean towards", "hang down over", "overhang", in case it has to do with this).
 
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