Define a function as result of "integrate"

Hello,

Maxima is wonderful, and everyone on the project
has my heartfelt thanks. Here is a simple question.

I want to define a function as a definite integral
with a variable upper limit. What I want is to carry
out the integration once, in order to get a form
which can then be evaluated repeatedly. 
However, what appears to happen is that the 
integrate function is not executed until the
definite integral is evaluated. Specifically, I have:

  erfc(t) := 1-erf(t);
  S(t) := erfc(log(t)/sqrt(2))/2;
  SI(t) := integrate(S(u), u, 0, t);

When I enter "integrate(S(u), u, 0, t);", an 
expression in terms of erf and log is the result.
Great! But when I say "SI(t) := integrate(S(u), u,
0, t);" the integral is not computed -- it seems
that SI is defined in terms of "integrate", and 
not in terms of the resulting expression. (The
integral appears to be postponed til I say "SI(t);"
or "SI(3);" or something like that.)

Of course, I can simply copy and paste the 
expression -- "SI(t) := (SQRT(%E)*ERF((SQRT(2) ...."
which works fine, but I wonder if there is another
way to get the same effect.

I've read about nouns and verbs, "ev", functions,
and so on, but I can't seem to find a function
definition that's equivalent to copy & paste on
the result returned by "integrate".

Thanks for any light you can shed on this problem.

Robert Dodier


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