Define Atom of domain relational calculus

An atom has the outline < x₁, x₂, ... , x_n > -> r , in which r stands for a relation on n attributes and x₁, x₂, ... , x_n are the domain variables or domain constraints. 

 x Θ y , in which x and y are domain variables and Θ is the comparison operator (<, >,  <,  >, =, ≠).  It is needed that x and y have domains which can be compared by Θ. 

 x Θ c, in which x is a domain variable, Θ is a comparison operator, and c is a constraint in the domain of attributes for which x is a domain variable.