truncated regressor

which is clear to me: doing lm(y ~ a + I(b+c)) is similar to

	d <- b + c
	lm(y ~ a + d)

But what if the expression in I() is a logical? After playing a moment, I
found out that a formula can include a logical variable, and treats it as a
factor:

x <- runif(50, 0, 50)
y <- 2*x + rnorm(50, 0, 5)
lm(y ~ x < 25)

Call:
lm(formula = y ~ x < 25)

Coefficients:
(Intercept)       x < 25  
      78.24       -53.92  

I should hope that it is being converted to *numeric* (TRUE --> 1),
not factor, otherwise there'd be all sorts of trouble with contrasts.
This is a general feature: TRUE + 2 == 3 for instance, and (x>0)-(x<0)
is the sign function. 

[Easiest way of checking is to compare

lm(y ~ I(x < 25) - 1) 

with 

lm(y ~ factor(x < 25) - 1)   

]