I made up the Collapsing-E notation (&E) since October 2022 for extending the Extended Cascading-E (xE^) and Hyper-Extended Cascading-E (#xE^) notations by Sbiis Saibian, and that's a continuation of the Extensible-E system after the Hyper-Extended Cascading-E notation has been concluded.
It extends upon the Hyper-Extended Cascading-E notation by applying Ordinal Collapsing Function (OCF)-like fundamental sequence diagonalization to hyperions using even larger ones, based on the ampersand symbol (&), which is analogous to the first uncountable ordinal (Ω) in the OCF. In that case, #{&}# is the simplest level of the Collapsing-E notation, which diagonalizes over the limit of the Hyper-Extended Cascading-E notation. For instance, #{&}# in this notation has the same fundamental sequence as {#,#,1,2} in the Hyper-Hyper-Extended Cascading-E notation (##xE^), while (#{&}#){&}# and {{#,#,1,2},#,1,2} are slightly different.
The limit of its growth rate is comparable to the Bachmann-Howard ordinal (f_{φ0(Ω_2)}(n)) in the fast-growing hierarchy, which is far larger than the former limit of the #xE^'s growth rate of the Ackermann ordinal (f_{φ(1,0,0,0)}(n)).