d_ with mixed-dimension indices contracts

[jodavies]

d_ with mixed-dimension indices should probably not contract with anything, but rather just sit "inert"? For eg,

Index i4;
Symbol n;
Dimension n;
Index in;

Local test1 = d_(i4,in)*d_(in,i4);
Local test2 = d_(in,i4)*d_(i4,in);
Local test3 = d_(in,i4)*d_(in,i4);
Local test4 = d_(i4,in)*d_(i4,in);

Print +s;
.end

yielding

   test1 =
       + n
      ;

   test2 =
       + 4
      ;

   test3 =
       + 4
      ;

   test4 =
       + n
      ;