SEM is always tagged C.
Every clause containing a SEM complementizer also contains a gap.
SEM comparatives do not have an introducing P head: they are just CP-CMP, with SEM in C and a gap of the appropriate kind in Spec(CP). Note that this is not the same as the treatment of "as"-comparatives in the PPCME2, PPCEME, even though SEM is sometimes translated similarly to English "as".
When SEM introduces a relative clause, see CP-REL.
When sem means 'as' or 'like', as in sem barn 'as a child' and results in a gapped structure, treat sem as a complementizer that introduces a CP-ADV clause which does not have to contain a subject, cf. the example below:
(IP-SUB (NP-SBJ (PRO-N hann-hann)) (MDDS skyldi-skulu) (NP-OB1 (PRO-D henni-hún)) (VB fylgja-fylgja) (CP-ADV (WADVP-4 0) (C sem-sem) (IP-SUB (ADVP *T*-4) (NP-OB1 (NP-POS (PRO-D sinni-sinn)) (N-D móður-móðir)))))))))
See also the discussion of SEM in CP-CMP.
We treat SEM AÐ as a single C, see Splitting and joining words.
(ADVP (ADVR svo-svo) (CP-CMP-SPE (WADVP-1 0) (C (C21 sem-sem) (C22 að-að)) (IP-SUB-SPE (IP-SUB-SPE (ADVP *T*-1) (VBPI þekkir-þekkja) (NP-OB1 (PRO-A mig-ég)) (NP-SBJ (NP-POS (PRO-N minn-minn)) (N-N faðir-faðir)))