Strictly Precede and Step Condition and not Precede implies Joins are equal

Theorem
Let $\left({S, \vee, \preceq}\right)$ be a join semilattice.

Let $p, q, u \in S$ be such that:


 * $p \prec q$ and $\left({\forall s \in S: p \prec s \implies q \preceq s}\right)$ and $u \npreceq p$

Then:


 * $p \vee u = q \vee u$

Proof
From the definition of join, it is required to prove that:
 * $\forall s \in S: p \preceq s \land u \preceq s \implies q \vee u \preceq s$

Let $s \in S$ be such that:
 * $p \preceq s$ and $u \preceq s$

We have:
 * $p \ne s$

By definition of strictly precede:
 * $p \prec s$

By assumption:
 * $q \preceq s$

Thus by definition of the join $q \vee u$:
 * $q \vee u \preceq s$

and hence, as desired:


 * $q \vee u = p \vee u$