/** Basic types used by various Penman modules. */
export type Variable = string;
export type Constant = string | number | null;
export type Role = string;
export type Branch = [Role, any];
export type Node = [Variable, Branch[]] | [Variable];
export type Target = Variable | Constant;
/** Represents a relation between nodes or between a node and a constant. */
export type Triple = [source: Variable, role: Role, target: Target];
export type Triples = Triple[];
/**  A relation indicating the concept of a node. */
export type Instance = [source: Variable, role: Role, target: Constant];
/** A relation between nodes. */
export type Edge = [source: Variable, role: Role, target: Variable];
/** A relation between a node and a constant. */
export type Attribute = [source: Variable, role: Role, target: Constant];