You cannot dynamically call a function passed as a value if it depends on aspects of the context in order to execute. Only built-in functions can depend on the context in this way.
Response
You might be able to use another version of the same function where you can explicitly provide the information otherwise used from the context. Or you might want to wrap your call to the function in an explicit inline function such as function($a,$b) { starts-with($a,$b) }.
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