p1  ::= 'the morning star' equals 'the planet venus'
    p2  ::= 'the evening star' equals 'the planet venus'
    p3  ::= 'the morning star' equals 'the evening star'
    
    (+) ::= an operator that means 'two things which equal 
            the same thing, equal each other'

    =>  ::= an operator which means 'this rule produces this result'