Add fix() and unfix() functions for parameter fixing and unfixing

Author: sunxd3

src/abstractprobprog.jl

@@ -62,6 +62,46 @@ should hold for generative models `m` and arbitrary `obs`.
 """
 function condition end
 
+"""
+    fix(model, params)
+
+Fix the values of parameters specified in `params` within the probabilistic model `model`. 
+This operation is equivalent to treating the fixed parameters as being drawn from a point mass 
+distribution centered at the values specified in `params`.
+
+Conceptually, this is similar to Pearl's do-operator in causal inference, where we intervene 
+on variables by setting them to specific values, effectively cutting off their dependencies 
+on their usual causes in the model.
+
+The invariant
+
+```
+m == unfix(fix(m, params))
+```
+
+should hold for any model `m` and parameters `params`.
+"""
+function fix end
+
+
+"""
+    unfix(model)
+
+Remove any fixed parameters from the model `model`, returning a new model without the fixed parameters.
+
+This function reverses the effect of `fix()` by removing parameter constraints that were previously set.
+It returns a new model where all previously fixed parameters are allowed to vary according to their 
+original distributions in the model.
+
+The invariant
+
+```
+m == unfix(fix(m, params))
+```
+
+should hold for any model `m` and parameters `params`.
+"""
+function unfix end
 
 """
     rand([rng=Random.default_rng()], [T=NamedTuple], model::AbstractProbabilisticProgram) -> T