Question: Meaning of some variables Maple give me back when I use DE solver

If I try to solve the following differential equation with simbolic parameters:

IC := p(0) = condizIni

ode := diff(p(t), t) = (QIn-QOut-derV)/((1/Boil+1/Bwall+percair/(m*p(t)))*V)

func := rhs(dsolve({IC, ode}))

the result I get from Maple12 have the following member inside it:

LambertW(_Z7, _Z)

but I do not know what are _Z7 and _Z.

Where I can find the expression of these parameters?

Why Maple added these parameters?

Thank you in advance for your help!


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