1 | initial version |

The problem arises from the fact that matrices with symbolic expressions are not "symbolic expressions" themselves:

```
sage: B = matrix([[u(k),v(q)],[v(q),u(k)]])
sage: type(B)
sage.matrix.matrix_symbolic_dense.Matrix_symbolic_dense
sage: type(u)
sage.symbolic.function_factory.NewSymbolicFunction
sage: type(u(q))
sage.symbolic.expression.Expression
```

and hence, trying to turn it into a CallableSymbolicExpression by itself fails.
Possibly the easiest thing is to just live with `B`

. You can still substitute values, as long as you're explicit about the substitutions:

```
sage: B(k=17,q=sin(q))
[ u(17) v(sin(q))]
[v(sin(q)) u(17)]
```

The nice thing is that you can do matrix algebra with these:

```
sage: H=B(k=k,q=q)*B(k=q,q=q)
sage: H
[ u(k)*u(q) + v(q)^2 u(k)*v(q) + u(q)*v(q)]
[u(k)*v(q) + u(q)*v(q) u(k)*u(q) + v(q)^2]
sage: H(k=sin(17),q=pi)
[ u(pi)*u(sin(17)) + v(pi)^2 u(pi)*v(pi) + u(sin(17))*v(pi)]
[u(pi)*v(pi) + u(sin(17))*v(pi) u(pi)*u(sin(17)) + v(pi)^2]
```

Presently, "callable" symbolic expressions and "symbolic" matrices are two extensions built on top of symbolic expressions and they don't mix very well.

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