How the new version achieves this:diff(x(t)*y(t)) in the latest version?

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How the new version achieves this: diff(x(t)*y(t)) in the latest version?

sage: t = var(’t’)
sage: x = function(’x’, t) 
sage: y = function(’y’, t) 
sage: diff(x(t)*y(t),t)
TypeError: Substitution using function-call syntax and unnamed arguments
has been removed. You can use named arguments instead, like EXPR(x=..., y=...)