# Expanding a bivariate exponential generating function, part II

From FrédéricC's answer to question 66860 I learned the following method (needs SageMath >= 9.8):

```
def egfExpand(egf, size):
y = polygen(QQ, "y")
x = LazyPowerSeriesRing(y.parent(), "x").gen()
return [list(egf(x, y)[n] * factorial(n)) for n in range(size)]
def f(x, y):
return exp(x * y) * exp(x)
#return exp(x * y) * hypergeometric((), (), x)
egfExpand(f, 10)
[[1],
[1, 1],
[1, 2, 1],
[1, 3, 3, 1],
[1, 4, 6, 4, 1],
...]]
```

But when I work with hypergeometric functions I get the error message:

```
cannot coerce arguments: no canonical coercion from Lazy Taylor Series Ring in x over Univariate Polynomial Ring in y over Rational Field to Symbolic Ring
```

The simplest example is when using the second line as definition of f(x, y) (the result should be identical).

What can I do?