In this sentence, the word "simmered" can be classified as a reduced relative clause.
What is a reduced relative clause?
This refers to a clause that has been reduced, which means some words of it had been omitted, which can create an ambiguous meaning. The most common is this clauses do not include words such as:
Why is "simmered" a reduced relative clause?
Because the complete clause would be "the stew that simmered in the stove" but this has been reduced to "simmered in the stove".
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