newbie - difficulty calling user defined function from by()

Hi Folks:
 
I'm new to R and am having trouble calling a user-defined function
within the by() function.  I have checked on-line help and the R
documentation to no avail.  I have a data frame with a sample subset
represented here:
 

example.sample

   ACCT_GROUP_DIM_KEY   MV_BASE TOT_DEBT TOT_EQTY
1        555586574850 1082576.3   685.00  2422.50
2        555586574850 1032994.2  2444.00  1724.00
3        555586574850  620471.0  3494.84  2712.59
4        555586574850  225047.4   592.21   514.17
5        555586574850  353844.5    48.25   396.28
6        555586574850  164043.6     0.00   204.14
7        555586574850  115792.4   436.25   669.60
8        555586574850 1299872.0  1794.21  1509.21
9        555586574851  100000.0     1.00   500.00
10       555586574851  100001.0     1.01   501.00

I am using the by() function to apply an aggregation on rows grouped by
the key value.  A simple mean aggregation works as seen here:

by (example.sample[, 2:4], example.sample["ACCT_GROUP_DIM_KEY"], mean)

ACCT_GROUP_DIM_KEY: 555586574850
   MV_BASE   TOT_DEBT   TOT_EQTY 
611830.169   1186.845   1269.061 
------------------------------------------------------------------------
------------------------- 
ACCT_GROUP_DIM_KEY: 555586574851
   MV_BASE   TOT_DEBT   TOT_EQTY 
100000.500      1.005    500.500 

I have defined a function that calculates a metric I want to apply
against each group.  The function is:

debt.eq

function (x) {sum(
(x["TOT_DEBT"]/x["TOT_EQTY"])*(x["MV_BASE"]/sum(x["MV_BASE"]))*100)}

When I call the function without any grouping I get:

debt.eq (example.sample)

[1] 88.1984

When I try to call my user-defined function in the by() function I get
the following error:

by (example.sample[, 2:4], example.sample["ACCT_GROUP_DIM_KEY"],

debt.eq(example.sample))
Error in FUN(X[[1L]], ...) : could not find function "FUN"

What is it I am doing wrong/missing here?  Thanks in advance,

John