remove all terms with interaction factor in formula

Hi Folks,

I'm trying to find a way to remove all terms in a formula that contain a
particular interaction.

For example, in the formula below, I'd like to remove all terms that
contain the b:c interaction.

attributes(terms( ~ a*b*c*d))$term.labels

 [1] "a"       "b"       "c"       "d"       "a:b"     "a:c"
 [7] "b:c"     "a:d"     "b:d"     "c:d"     "a:b:c"   "a:b:d"
[13] "a:c:d"   "b:c:d"   "a:b:c:d"

My eventual use is to fit models with the reduced formulas.

For example:

my_df = data.frame( iv = runif(100), a=runif(100), b=runif(100),

c=runif(100), d=runif(100))

lm(iv ~ a*b*c*d, data=my_df)

I can remove particular terms with update(), but I don't see a way to
remove all terms that contain a given combination of factors.

Any help would be greatly appreciated.  Thanks!

Allie