Atoms

likes(john,mary).
In the above fact john and mary are two atomes.
Atoms are usally made from letters and digits with lowercase characters.
Atoms can also be legally made from symbols. The followings are legal atoms :
atoms
hello
zz42
two_words
====>
'two words'


The followings are not legal atoms :
Hello
4hello
_Hello
two words
two-word
The fact likes(john,mary). say that there is a relation between john and mary. It can be read as either john likes mary or mary likes john.