Docs: Add comprehensive documentation for BasicSymbolic hash consing

src/types.jl

@@ -318,6 +318,55 @@ end
 
 wvd = WeakValueDict{UInt, BasicSymbolic}()
 
+"""
+$(TYPEDSIGNATURES)
+
+This function implements hash consing for `BasicSymbolic` objects, ensuring that only one 
+unique instance of a given symbolic expression exists in memory. 
+
+Hash consing, also known as the flyweight pattern, is a powerful optimization technique 
+that can significantly reduce memory consumption and speed up symbolic computations by 
+eliminating duplicate objects.
+
+# how it works
+
+1. **Hashing:** The input `BasicSymbolic` object `s` is hashed using `hash2` to generate a 
+    unique key.
+2. **Lookup:** The `wvd` dictionary, a 
+    [`WeakValueDict`](https://github.com/twavv/WeakValueDicts.jl), is checked to see if an 
+    object with the same hash already exists.
+3. **Return Existing Object:** If an object with the same hash is found and it `isequal` 
+    to `s`, the existing object is returned, avoiding the creation of a duplicate.
+4. **Handle Collisions and Return New Object:** If a hash collision occurs (two objects 
+    with different symbolic expressions having the same hash), a warning is issued to alert 
+    the user. The new object `s` is then returned and not stored in the `wvd` dictionary.
+
+# Benefits of Hash Consing
+
+- **Reduced Memory Consumption:** By eliminating duplicate symbolic objects, hash consing 
+    significantly reduces memory usage.
+- **Faster Equality Checks:**  Checking for equality between symbolic expressions becomes a 
+    simple pointer comparison, leading to speed-ups in algorithms that rely on equality 
+    checks.
+- **Implicit Common Subexpression Elimination (CSE):** Hash consing inherently performs 
+    CSE. Since each subexpression is constructed only once, redundant computations are 
+    avoided.
+- **Accelerated Runtime Code Generation:** In scenarios involving runtime code generation, 
+    hash consing reduces the number of unique symbolic expressions that need to be compiled.
+
+# The `wvd` Dictionary (WeakValueDict)
+
+The `wvd` dictionary uses weak references to its values. This means that if a symbolic 
+object loses all its strong references elsewhere in the code, it becomes eligible for 
+garbage collection, even if it's still present in the `wvd` dictionary. 
+
+# Example
+```julia
+expr1 = BasicSymbolic(...)
+expr2 = BasicSymbolic(...) # the same mathematical expression
+expr1 === expr2 # expr1 and expr2 refer to the same object in memory
+```
+"""
 function BasicSymbolic(s::BasicSymbolic)::BasicSymbolic
     h = hash2(s)
     t = get!(wvd, h, s)