Sin should only evaluate inexact numbers, and expressions that there
is an exact value for.

That is (Sin 1) should be (Sin 1).
(Sin 1.0) should evaluate.

For syntax error that reports an error in column 0 suggest that a semi colon is missing.

In: x^(-1)_(-9+x^2)
Out: x^(-1)_Plus(-9,x^2)

Currently every value is run through unparse (which returns a string) prior to printing it in the REPL. Values such as pict structures (from slideshow/picts) can not be converted into strings. The return type of unparse and friends thus needs to change into something that can support both strings, picts and other snips.

Problem: 1+a + (-1)*(1+a) + x

Solution? Distribute -1 over sums in expand-factor.

Improve this interaction:

> (b-c)/a
a^(-1)*(b-c)

1=1

f

Yikes.

The program:
#lang bracket
DeclareVars(x,a);
Expand(((x-1)^a)/(1-x));
Outputs:
(-1+x)^a/1-x

The parentheses are missing in the denominator.

x^-1
Syntax error

Should unary minus be accepted?
Should unary minus have a special symbol as in NSpire?

Unparse ought to handle function values.
From a bracket repl:

List(Sin)
. . ../unparse.rkt:77:13: string-append: contract violation
expected: string?
given: #procedure:Sin

Currently the syntax check button in DrRacket does not show any arrows.

This repo is missing a LICENSE file.

If you care, Racket itself is released under the LGPL: https://github.com/plt/racket/blob/master/racket/src/COPYING_LESSER.txt