An attribute is declared by writing its name and type, separated
by a colon.
A declaration has the form name: TYPE, such as balance: REAL
or age: INTEGER.
An attribute name is a full word, written in lower case.
A type is the name of a class; it is a full word, written in upper
case.
Multiple attributes can be declared in a single declaration; the names
are separated by commas.
A complex declaration has the form name, name: TYPE, such as
my_age, your_age: INTEGER