notes

ng a predicate that does not exist 

check that variables start with upper case

check that predicate name starts with lower case letter or _ underscore
predicates in the right hand side are pre-defined

type Obj = String — lower case
type NumArg = Int 

data Atoms = Var VarName | Str String | Num Int | [T] | _ 
data Predicate = Unary Name Atoms | Binary Name Atoms Atoms
data NewPredicate = Unary Name NumArg
data DefinedPredicate = Unary Name NumArg

ValExp =  LT _ _ | LTE _ _ | GT _ _ | GTE _ _  | EQ _ _  | Add _ _ | Mult _ _ | Div _ _ | Sub _ _ 
data Exp = Val ValExp | P Predicate

type VarName = String — Upper case

type Head = Predicate
type Body = Pred Predicate 
Conj Exp Exp
| Disj  Exp Exp 
| Is VarName Exp 

data  Command = 
Pred2 Predicate 
| Rule Head  Body

question: should we do run time checking