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+
+lexing/parsing options:
+- reuse old spectrum code as-as
+- modify spectrum code as iterator
+- nom (from s-expr example)
+
+sexpr_expr
+    Integer(i64)
+    Float(f64)
+    String(String)
+    Boolean(bool)
+    Symbol(String)
+    List: vec of expr
+
+later add:
+    Vector
+    Quote
+
+
+--- basic CAS
+
+canonical_expr
+    
+    Number
+      Integer
+      Rational
+        denominator always positive
+        both are non-zero integers
+      Float
+      Constant (pi, etc)
+    Sum
+      at least two members
+      no members are Sum
+      at most one member is a number
+    Product
+      at least two members
+      no members are Product
+      at most one member is a number
+    Power
+    Factorial
+      unary
+      simplify if operand is a positive integer
+    UnaryFunction
+      eg, sin, cos
+
+simple test cases:
+
+  (+ 1 2 3) => 6
+  (/ c (^ d 2)) => (* c (^ d -2))
+  (- a (* 2 b)) => (+ a (* -2 b))
+  (- a b) => (+ a (* -1 b))
+  (* a (* b c)) => (* a b c)
+
+- parse sexpr into canonical expression
+- rational number
+
+more expr:
+    Complex (split up Number)
+    Limit
+    Series
+    Equation(left, right)
+    Vector
+    Operator (?)
+    Derivative (?)
+    Integral (?)