Consider the following two definitions, $D_i$ and $D_j$:
$ \bar{x} \; : \; \bar{A} \; \triangleright \; a(\bar{x}) \; := \; K : L$
$ \bar{y} \; : \; \bar{B} \; \triangleright \; b(\bar{y}) \; := \; M : N$
Let $Δ ; Γ ⊢ U : V$ and assume that $D_i$ and $D_j$ are elements of the list $Δ$, where $D_i$ precedes $D_j$.
(a) Describe exactly where the constant $a$ may occur in $D_i$ and $D_j$.
(b) Describe where the constant $b$ may occur in $Δ$.
(a) Since $D_i$ defines the constant $a$, it can only occur once in $D_i$ at the position of the definiendum. It cannot occur in the definiens, because we don't allow self-referencing definitions, which means it is not in $K$ nor in $L$. It cannot occur in $\bar{x}$ because it hasn't been defined at this point.
Since $D_j$ occurs after $D_i$, the constant $a$ can occur anywhere in $D_j$, except as the definiendum because we can't redefine the constant $a$. So $a$ can occur in $\bar{B}$, $M$ and $N$.
(b) The constant $b$ can only occur after it has been defined. So it cannot occur anywhere in $\Delta$ before $D_j$, and that means it cannot occur in $D_i$ which occurs before $D_j$.
