@Override
    public boolean equals(Object obj) {
        if (!(obj instanceof ForEachExpression rhs)) return false;
        if (!expression.equals(rhs.expression)) return false;
        return true;
    }
Verifies that each struct field is compatible with the expression.
This is symmetric in both verification directions since the expression just need to be compatible with
all the struct fields.
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/expressions/ForEachExpression.java