term_vars(u))
i:
. i < ||v||+1 & (x term_vars([u / v][i])) reduce(
t,vs. term_vars(t) @ vs;nil;v))7.
i:. i < ||v|| & (x term_vars(v[i])) i:. i < ||v||+1 & (x term_vars([u / v][i])) reduce(t,vs. term_vars(t) @ vs;nil;v)) 
(i:. i < ||v|| & (x term_vars(v[i])))
term_vars(u) @ reduce(t,vs. term_vars(t) @ vs;nil;v))
i:. i < ||v||+1 & (x term_vars([u / v][i]))
| 1 | 7. (x term_vars(u)) i:. i < ||v||+1 & (x term_vars([u / v][i])) |
| 2 | 6. (x reduce(t,vs. term_vars(t) @ vs;nil;v)) 7. i:. i < ||v|| & (x term_vars(v[i])) i:. i < ||v||+1 & (x term_vars([u / v][i])) |
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