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Terminal: run. Derivation Graphs: (left-to-right) compact / full; (top-down) full / compact.

Help: blue = parse inside. Abbr.: WS: Workspace; Initial WS: initial heads for Merge after LEX lookup.

WS n, n = 1,2,... for intermediate stages after Initial WS. Final WS: when WS items converge on a single SO.
Bracketing: {...} (Merge). {...} (FormSet). At Spellout, {...} indicates free word order. [word] indicates word is optionally present.
Head Hd_{selection/features}, e.g. p_θ = predicate head p that seeks a θ-relevant (θR) IA from the WS. p_θ is a θ-seeker (θS).
v_see:θ:pst = head v has complement see, a θR EA from the WS and TNS. v is also a θS.
INFL seeks θR from its complement (not the WS).
Complementizer C comes in different flavors wrt. Force, .e.g. declarative C, interrogative C_Q and relative clause head C_rel.
Features, e.g. TNS pres/pst or DET the/a (shorthand for a bundle of semantic features).
Note: φ-features, e.g. 1sg, appear only at Spellout. Some words do not project, turn into features, e.g. determiners.

Words: llegaron dos trenes

Initial WS 1: trenes_dos llegar_θ v_llegar:_pst INFL_v:_2pl

WS 1: {llegar_θ, trenes_dos} v_llegar:_pst INFL_v:_2pl

WS 2: {v_llegar:_pst, {llegar_θ, trenes_dos}} INFL_v:_2pl

Initial WS 2: trenes_dos llegar_θ v_llegar:_pst INFL_v:_3pl

WS 1: {llegar_θ, trenes_dos} v_llegar:_pst INFL_v:_3pl

WS 2: {v_llegar:_pst, {llegar_θ, trenes_dos}} INFL_v:_3pl

WS 3: {INFL_v:_3pl, {v_llegar:_pst, {llegar_θ, trenes_dos}}}

Final WS: {C, {INFL_v:_3pl, {v_llegar:_pst, {llegar_θ, trenes_dos}}}}

INT/EXT: {C, {INFL_v:_3pl, {v_llegar:_pst, {llegar_θ, trenes_dos}}}}

EXT: dos trenes 3pl-m pst llegar

Spellout: dos trenes llegaron

Blocked: inconsistent with input!

EXT: 3pl-m pst llegar dos trenes

Spellout: llegaron dos trenes

Parse found: 3pl-m pst llegar dos trenes