Compares an object with another and returns
TRUE if they are equal.
The equal property must be
1) reflexive, i.e.
equals(o1,o1) should be
5) consistent, i.e.
equals(o1,o2) should return the same
result on multiple invocations as long as nothing has changed.
identical() is used.
# S3 method for default equals(object, other, ...)
Objects to be compared.