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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: did a train arrive

Initial WS 1: arrive_θ v_arrive:_pst:_ext(do) train_a INFL_v

WS 1: {arrive_θ, train_a} v_arrive:_pst:_ext(do) INFL_v

WS 2: {v_arrive:_pst:_ext(do), {arrive_θ, train_a}} INFL_v

WS 3: {INFL_v, {v_arrive:_pst:_ext(do), {arrive_θ, train_a}}}

Final WS: {C, {INFL_v, {v_arrive:_pst:_ext(do), {arrive_θ, train_a}}}}

INT/EXT: {C, {INFL_v, {v_arrive:_pst:_ext(do), {arrive_θ, train_a}}}}

EXT: a train 3sg pst arrive

Spellout: a train arrived

Blocked: inconsistent with input!

EXT: there 3sg pst arrive a train

Spellout: there arrived a train

Blocked: inconsistent with input!

Final WS: {C_Q:_{aux(null,pst,arrive)}, {INFL_v, {v_arrive:_pst:_ext(do), {arrive_θ, train_a}}}}

INT/EXT: {C_Q:_{aux(null,pst,arrive)}, {INFL_v, {v_arrive:_pst:_ext(do), {arrive_θ, train_a}}}}

EXT: null pst arrive →→ a train 3sg pst arrive

Spellout: 3sg pst do a train arrive

Spellout: did a train arrive

Parse found: null pst arrive →→ a train 3sg pst arrive

EXT: null pst arrive →→ there 3sg pst arrive a train

Spellout: 3sg pst do there arrive a train

Spellout: did there arrive a train

Blocked: inconsistent with input!