You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: GB_spec_mxm.m
    path of file: ./repos/SuiteSparse/GraphBLAS/Test
    
    the code of the file until where you have to start completion: function C = GB_spec_mxm (C, Mask, accum, semiring, A, B, descriptor)
%GB_SPEC_MXM a mimic of GrB_mxm
%
% Usage:
% C = GB_spec_mxm (C, Mask, accum, semiring, A, B, descriptor)
%
% Computes C<Mask> = accum(C,T), in GraphBLAS notation, where T =A*B, A'*B,
% A*B' or A'*B'.  The matrix C is returned as a struct with C.matrix being the
% values of the matrix, C.pattern the 'nonzero' pattern, and C.class the type
% of the matrix.  A and B can be plain matrices on input, or they can be
% structs like C.  See GB_spec_matrix.m for more details.  See
% GB_spec_transpose.m for a description of the input parameters, except for
% semiring; see GB_spec_semiring for a description of the semiring.
%
% The semiring defines the multiply and add operators and the additive
% identity.  The matrix multiplication T = A*B is then defined as:
%
%   [m s] = size (A) ;
%   [s n] = size (B) ;
%   ztype, xtype, ytype: the types of the multiply operator
%   T = identity (m,n,ztype) ; where identity is defined by semiring.add
%   for j = 1:n
%       for i = 1:m
%           for k = 1:s

    
function C = GB_spec_mxm (C, Mask, accum, semiring, A, B, descriptor)
%GB_SPEC_MXM a mimic of GrB_mxm
%
% Usage:
% C = GB_spec_mxm (C, Mask, accum, semiring, A, B, descriptor)
%
% Computes C<Mask> = accum(C,T), in GraphBLAS notation, where T =A*B, A'*B,
% A*B' or A'*B'.  The matrix C is returned as a struct with C.matrix being the
% values of the matrix, C.pattern the 'nonzero' pattern, and C.class the type
% of the matrix.  A and B can be plain matrices on input, or they can be
% structs like C.  See GB_spec_matrix.m for more details.  See
% GB_spec_transpose.m for a description of the input parameters, except for
% semiring; see GB_spec_semiring for a description of the semiring.
%
% The semiring defines the multiply and add operators and the additive
% identity.  The matrix multiplication T = A*B is then defined as:
%
%   [m s] = size (A) ;
%   [s n] = size (B) ;
%   ztype, xtype, ytype: the types of the multiply operator
%   T = identity (m,n,ztype) ; where identity is defined by semiring.add
%   for j = 1:n
%       for i = 1:m
%           for k = 1:s
%               if (A (i,k) and B (k,j) are 'nonzero'
%                   aik = cast (A(i,k), xtype)
%                   bkj = cast (B(k,j), ytype)
%                   T (i,j) = add (T (i,j), multiply (aik, bkj))
%               end