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NullReferenceException when using Power operator

See original GitHub issue

When raising a Variable to a power, using the overloaded XOR operator (^) I get a NullReferenceException.

This is minimal working example to reproduce:

using System;
using Microsoft.ML.Probabilistic.Models;

internal static class Program
{
    private static void Main()
    {
        var a = Variable.Observed(3.14);
        var b = Variable.Observed(2.0);

        var c = a ^ b;
        
        var engine = new InferenceEngine();
        var res = engine.Infer(c);
        
        Console.WriteLine(res.ToString());
    }
}

And this is the stack trace:

Compiling model...
Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.ML.Probabilistic.Compiler.Transforms.StocAnalysisTransform.InferMarginalPrototype(IMethodInvokeExpression imie, IExpression target, Object targetDecl)
   at Microsoft.ML.Probabilistic.Compiler.Transforms.StocAnalysisTransform.InferMarginalPrototype(IAssignExpression iae, Boolean targetHasLiteralIndices)
   at Microsoft.ML.Probabilistic.Compiler.Transforms.StocAnalysisTransform.ConvertAssign(IAssignExpression iae)
   at Microsoft.ML.Probabilistic.Compiler.Transforms.ShallowCopyTransform.ConvertExpression(IExpression expr)
   at Microsoft.ML.Probabilistic.Compiler.Transforms.ShallowCopyTransform.ConvertExpressionStatement(IExpressionStatement ies)
   at Microsoft.ML.Probabilistic.Compiler.Transforms.ShallowCopyTransform.ConvertStatement(IStatement ist)
   at Microsoft.ML.Probabilistic.Compiler.Transforms.ShallowCopyTransform.ConvertStatements(IList`1 outputs, IEnumerable`1 inputs)
   at Microsoft.ML.Probabilistic.Compiler.Transforms.ShallowCopyTransform.DoConvertMethod(IMethodDeclaration md, IMethodDeclaration imd)
   at Microsoft.ML.Probabilistic.Compiler.Transforms.ShallowCopyTransform.ConvertMethod(IMethodDeclaration imd)
   at Microsoft.ML.Probabilistic.Compiler.Transforms.ShallowCopyTransform.ConvertMethods(ITypeDeclaration td, ITypeDeclaration itd)
   at Microsoft.ML.Probabilistic.Compiler.Transforms.ShallowCopyTransform.ConvertType(ITypeDeclaration itd)
   at Microsoft.ML.Probabilistic.Compiler.CodeTransformer.TransformToDeclaration(ITypeDeclaration typeDecl)
   at Microsoft.ML.Probabilistic.Compiler.TransformerChain.TransformToDeclaration(ITypeDeclaration itd, AttributeRegistry`2 inputAttributes, Boolean trackTransform, Boolean showProgress, List`1& warnings, Boolean catchExceptions, Boolean treatWarningsAsErrors)
   at Microsoft.ML.Probabilistic.Compiler.ModelCompiler.GetTransformedDeclaration(ITypeDeclaration itd, MethodBase method, AttributeRegistry`2 inputAttributes)
   at Microsoft.ML.Probabilistic.Compiler.ModelCompiler.CompileWithoutParams(ITypeDeclaration itd, MethodBase method, AttributeRegistry`2 inputAttributes)
   at Microsoft.ML.Probabilistic.Models.InferenceEngine.Compile()
   at Microsoft.ML.Probabilistic.Models.InferenceEngine.GetCompiledInferenceAlgorithm(Boolean inferOnlySpecifiedVars, IVariable var)
   at Microsoft.ML.Probabilistic.Models.InferenceEngine.Infer(IVariable var)
   at Program.Main()