Expression Trees and Lambdas

Question

In the following code, an Expression Tree is being constructed
from a Lambda Expression.

With regards to Expression Trees I know the common practice
is to build from the bottom up but reversed when coded i.e.

ParameterExpression 1st
MemberExpression 2nd
ConstantExpression 3rd
BinaryExpression 4th

LambdaExpression 5th - for executing and compiling to a delegate.

With expressions 1-4 being used to form the Body of the Expression Tree.

My question is from the code example below I cant seem
to see how the Expressions 1-4 are being used to form the Body?

It looks like only some are used?

Regards,

public static class PredicateBuilder

{

    private static readonly MethodInfo asNonUnicodeMethodInfo = 

                typeof(EntityFunctions).GetMethod("AsNonUnicode");

    private static readonly MethodInfo stringEqualityMethodInfo = 

                typeof(string).GetMethod("op_Equality");

    public static Expression<Func<TEntity, bool>> ContainsNonUnicodeString<TEntity>(

                IEnumerable<string> source, 

                Expression<Func<TEntity, string>> expression) 

    {

        if (source == null) throw new ArgumentNullException("source");

        if (expression == null) throw new ArgumentNullException("expression");

        Expression predicate = null;

        foreach (string value in source)

        {

            var fragment = Expression.Equal(

                expression.Body, 

                Expression.Call(null, 

                    asNonUnicodeMethodInfo, 

                    Expression.Constant(value, typeof(string))), 

                false, 

                stringEqualityMethodInfo);

            if (predicate == null)

            {

                predicate = fragment;

            }

            else

            {

                predicate = Expression.OrElse(predicate, fragment);

            }

        }

        return Expression.Lambda<Func<TEntity, bool>>(predicate,

            ((LambdaExpression)expression).Parameters);

    }

}