  The sequence A000975 in OEIS can be defined by $A_1=1$, $A_{n+1}=2A_n$ if $n$
is odd, and $A_{n+1}=2A_n+1$ if $n$ is even. This sequence satisfies other
recurrence relations, admits some closed formulas, and is known to enumerate
several interesting families of objects. We provide a new interpretation of
this sequence using a binary operation defined by $a\ominus b := -a -b$. We
show that the number of distinct results obtained by inserting parentheses in
the expression $x_0\ominus x_1\ominus \cdots\ominus x_n$ equals $A_n$, by
investigating the leaf depth in binary trees. Our result can be viewed as a
quantitative measurement for the nonassociativity of the binary operation
$\ominus$.
