  We show that in $\text{O}(D)$ invariant matrix theories containing a large
number $D$ of complex or Hermitian matrices, one can define a
$D\rightarrow\infty$ limit for which the sum over planar diagrams truncates to
a tractable, yet non-trivial, sum over melon diagrams. In particular, results
obtained recently in SYK and tensor models can be generalized to traditional,
string-inspired matrix quantum mechanical models of black holes.
