[Right triangle ⏵ may be expanded, down triangle ⏷ to close. Button = tree pop-up. × = close pop-up.]

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: there is arriving a train

Initial WS 1: train_a arrive_θ:_presp v_arrive v_v:_prog:_pres INFL_v:_3sg

WS 1: {arrive_θ:_presp, train_a} v_arrive v_v:_prog:_pres INFL_v:_3sg

WS 2: {v_arrive, {arrive_θ:_presp, train_a}} v_v:_prog:_pres INFL_v:_3sg

WS 3: {v_v:_prog:_pres, {v_arrive, {arrive_θ:_presp, train_a}}} INFL_v:_3sg

WS 4: {INFL_v:_3sg, {v_v:_prog:_pres, {v_arrive, {arrive_θ:_presp, train_a}}}}

Final WS: {C, {INFL_v:_3sg, {v_v:_prog:_pres, {v_arrive, {arrive_θ:_presp, train_a}}}}}

INT/EXT: {C, {INFL_v:_3sg, {v_v:_prog:_pres, {v_arrive, {arrive_θ:_presp, train_a}}}}}

EXT: a train 3sg pres be ing arrive

Spellout: a train is arriving

Blocked: inconsistent with input!

EXT: there 3sg pres be ing arrive a train

Spellout: there is arriving a train

Parse found: there 3sg pres be ing arrive a train

Final WS: {C_Q:_{aux(3sg,pres,be)}, {INFL_v:_3sg, {v_v:_prog:_pres, {v_arrive, {arrive_θ:_presp, train_a}}}}}

INT/EXT: {C_Q:_{aux(3sg,pres,be)}, {INFL_v:_3sg, {v_v:_prog:_pres, {v_arrive, {arrive_θ:_presp, train_a}}}}}

EXT: 3sg pres be →→ a train 3sg pres be ing arrive

Spellout: 3sg pres be a train ing arrive

Spellout: is a train arriving

Blocked: inconsistent with input!

EXT: 3sg pres be →→ there 3sg pres be ing arrive a train

Spellout: 3sg pres be there ing arrive a train

Spellout: is there arriving a train

Blocked: inconsistent with input!