M[x] returns the multiplicity of the member x of the MultiSet M.

M[x] := n sets the multiplicity of x in M to n.  Note that this is different from Insert( M, x=n ), which increases the multiplicity of x in M by n.

Note M[x] := 0 is equivalent to Remove( M, x ).

If M is a generalized MultiSet, n may be any real number.  Otherwise, it must be a non-negative integer.

$M≔\mathrm{MultiSet}\left(a=2\,b=5\right)$

$M≔\left\{\left[a\,2\right]\,\left[b\,5\right]\right\}$

$M\left[a\right]$

$2$

$M\left[b\right]≔7$

$M_b≔7$

$M\left[a\right]≔0$

$M_a≔0$

$M\left[c\right]≔4$

$M_c≔4$

$M$

$\left\{\left[b\,7\right]\,\left[c\,4\right]\right\}$

$N≔\mathrm{MultiSet}\left[\mathrm{generalized}\right]\left(\right)$

$N≔\varnothing$

$N\left[w\right]≔3.14159$

$N_w≔3.14159$

$N$

$\left\{\left[w\,3.14159\right]\right\}$