expr            : binary_op(s?) term

binary_op       : term (/[+-*/><]/|'=='|'!='|'>='|'<='|'and'|'or')

term            : unary_op   # Must be above id, array_ref & func_ref
                | func_call  # Must be above id, array_ref & func_ref
                | const      # Must be above id, array_ref & func_ref
                | array_ref  # Must be above id 
                | func_ref   # Must be above id 
                | id
                | parens

unary_op        : ('+'|'-'|'not') term

##</code><code>##

unary_op        : '+'   term
                | '-'   term
                | 'not' term

##</code><code>##

expr            : binary_op(s?) term

# Lowest precendance.
binary_op       : binary_op_2 /and|or/
...
binary_op_8     : binary_op_9 /[+-]/
binary_op_9     : term        /[*/]/
# Highest precendance.

term            : ...