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Hold[], Evaluate[] and so forth.


Dear MathGroup, I'm currently in a confused state regarding
the use of Hold[], Evaluate[] etc. Is there a good discussion
of this somewhere (apart from "Mma Book" which is a bit vague).

Specifically, I have the following problem: suppose I have a
string containing an expression. I want to get a exact copy
of that as an expression, without any evaluation. I can do:
ToHeldExpression[string]
but this puts a Hold[...] around the result. I can't see any
way of removing the Hold[] without the thing inside getting
evaluated.

On a more general level, is there a way to do an Evaluate[]
to some specified level of an expression, rather than 
recursing all the way down?

Thanks,
Phillip Kent
Transitional Mathematics Project
Imperial College 
London.







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