Correct use of Depends, Imports and ::

Dear R developers,

Taking advantage of the yesterday discussion about the use of 
Depends/Import/Suggests/Enhances, I would like to add a related question.

Let's assume, in the DESCRIPTION file of my package foo0, I have:

Depends: foo1
Imports: foo2

while in the NAMESPACE file of my package I have

importFrom("foo2", f2)


and within my package I use the following two external functions:

f1, from package foo1
f2, from package foo2


If I want to use the two previous functions within a function of my 
package foo0, is it correct to call them as follow ( assuming 'x' is the 
only argument of 'f1' and 'f2'):

f1(x)
foo2::f2(x)

OR should I use

foo1::f1(x)
f2(x)

OR

f1(x)
f2(x)

OR

something else (the correct way of doing it in R) ?



Finally, can I use the following call in the body of a function of my 
package foo0:

foo3::f3(x)

without declaring the package 'foo3' in the Imports section of my 
DESCRIPTION file ?


Thanks in advance for your help,


Mauricio Zambrano-Bigiarini

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Water Resources Unit
Institute for Environment and Sustainability (IES)
Joint Research Centre (JRC), European Commission
TP 261, Via Enrico Fermi 2749, 21027 Ispra (VA), IT
webinfo    : http://floods.jrc.ec.europa.eu/
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