Arguments of a function

Dear all,

I have a question about how to set arguments in my own function. For
example, I have a function that looks like this:

my.f <- function(a = x1, b = x2)
{
   x1 = equation 1
   x2 = equation 2
   x3 = equation 3
   y = a + b   
}

x1, x2, and x3 are temporary variables (intermediate results) calculated
from other variables within the funciton. I want to use two of these three
variables to calculate y, and write R script as below:

my.f(a = x1, b = x2)
 
or 

my.f(a = x2, b = x3)

The error information shows that: ?objects 'x1', 'x2', or 'x3' not found?. 

Can anybody help me solve this problem? Thanks in advance.

Lisa

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