-type boolean_op() :: 'and' | 'or'.

-type comparison_op() :: '<' | '<=' | '=' | '>=' | '>' | '<>'.

-type event() :: [{atom(), value()}].

-type exp_tree() ::
          {boolean_op(), exp_tree(), exp_tree()} |
          {comparison_op(), variable(), value()} |
          {inclusion_op(), variable(), [value(), ...]} |
          {null_op(), variable()}.

-type inclusion_op() :: in | notin.

-type null_op() :: null | notnull.

-type value() :: integer() | float() | binary().

-type variable() :: atom().