Domain of parsed forms which can be passed to the interpreter. This is also the interface between algebra code and facilities in the interpreter.

Test parse and 'unparse':

(1) -> parse("sin x + 1")$InputForm

(1)

unparse(parse("sin x + 1")$InputForm)

(2)

-- syntax error gives empty result
null? parse("sin(x + 1")$InputForm

(3)