oberonjs/doc/wiki/eberon-array-methods.md

indexOf method is applied to array to search for specified element:

elementIndex := array.indexOf(elementValueToSearch);

indexOf returns the index of the first element of array which value is equal to the value specified as the first argument. Its logic is equivalent to the following code:

elementIndex := 0;
WHILE (elementIndex < LEN(array)) & (array[elementIndex] # elementValueToSearch) DO
    INC(elementIndex)
END;
IF elementIndex = LEN(array) THEN
    elementIndex := -1
END;

That is a lot of code and it cannot be reused in a library (element type can vary). On the other side this operation is used pretty often so it was implemented as [[Eberon|eberon]] extension.

indexOf can be applied only if elements type can be used in relation operation, i.e. it cannot be applied to array of records or array of arrays.