Lambda returns a lambda primary node and true if that's the only child of the compound node.

Primary returns a primary node and true if that's the only child of the compound node.

Shape describes the shape of the compound node.

StringLiteral returns the value of a string literal and true if that's the only child of the compound node.

StringLiteralOrError is like StringLiteral, but returns an error suitable as a compiler error when StringLiteral would return false.