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Consider the definition

phi = M.scalar_field(function('phi')(*coord), name='phi')

There are three layers here:

• phi itself,
• inside it is the expression function('phi')(*coord),
• and inside that is the function function('phi').

If you want to call substitute_function on an expression, you should pass two functions.

To access the function inside phi you need to peel off the layers: phi.expr().operator().

So, you can do what you want as follows:

E[0,0].expr().substitute_function(phi.expr().operator(), F1.expr().operator())

You can also assign the function inside phi to a variable (before defining phi), so you can use that instead.

Consider the definition

phi = M.scalar_field(function('phi')(*coord), name='phi')

There are three layers here:

• the scalar field phi itself,
• inside it is the expression function('phi')(*coord),
• and inside that is the function function('phi').

If you want to call substitute_function on an expression, you should pass two functions.

To access the function inside phi you need to peel off the layers: phi.expr().operator().

So, you can do what you want as follows:

E[0,0].expr().substitute_function(phi.expr().operator(), F1.expr().operator())

You can also assign the function inside phi to a variable (before defining phi), so you can use that instead.