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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: they are planes

Initial WS 1: planes be_θ v_be:_θ:_pres INFL_v:_pl they

WS 1: {be_θ, planes} v_be:_θ:_pres INFL_v:_pl they

WS 2: {v_be:_θ:_pres, {be_θ, planes}} INFL_v:_pl they

WS 3: {they, {v_be:_θ:_pres, {be_θ, planes}}} INFL_v:_pl

WS 4: {INFL_v:_pl, {they, {v_be:_θ:_pres, {be_θ, planes}}}}

Final WS: {C, {INFL_v:_pl, {they, {v_be:_θ:_pres, {be_θ, planes}}}}}

INT/EXT: {C, {INFL_v:_pl, {they, {v_be:_θ:_pres, {be_θ, planes}}}}}

EXT: they 3pl pres be planes

Spellout: they are planes

Parse found: they 3pl pres be planes

WS 1: {be_θ, they} planes v_be:_θ:_pres INFL_v:_pl

WS 2: {v_be:_θ:_pres, {be_θ, they}} planes INFL_v:_pl

WS 3: {planes, {v_be:_θ:_pres, {be_θ, they}}} INFL_v:_pl

WS 4: {INFL_v:_pl, {planes, {v_be:_θ:_pres, {be_θ, they}}}}

Final WS: {C, {INFL_v:_pl, {planes, {v_be:_θ:_pres, {be_θ, they}}}}}

INT/EXT: {C, {INFL_v:_pl, {planes, {v_be:_θ:_pres, {be_θ, they}}}}}

Error: Case: object "they" not ACC

WS 1: {planes, they} be_θ v_be:_θ:_pres INFL_v:_pl

WS 2: {be_θ, {planes, they}} v_be:_θ:_pres INFL_v:_pl

WS 3: {v_be:_θ:_pres, {be_θ, {planes, they}}} INFL_v:_pl

WS 4: {{planes, they}, {v_be:_θ:_pres, {be_θ, {planes, they}}}} INFL_v:_pl

WS 5: {INFL_v:_pl, {{planes, they}, {v_be:_θ:_pres, {be_θ, {planes, they}}}}}

Final WS: {C, {INFL_v:_pl, {{planes, they}, {v_be:_θ:_pres, {be_θ, {planes, they}}}}}}

INT/EXT: {C, {INFL_v:_pl, {{planes, they}, {v_be:_θ:_pres, {be_θ, {planes, they}}}}}}

EXT: {planes,[and],they} 3pl pres be {planes,[and],they}

Blocked: EXT inconsistent!

Blocked: inconsistent with input!