Macro define_units_relationship 

macro_rules! define_units_relationship {
    { $exact:tt $with_approx:tt $with_correlation:tt, $unit1:ident 1 == $unit2:ident 1 * __ 1 } => { ... };
    { $exact:tt $with_approx:tt $with_correlation:tt, $unit1:ident 1 == $unit2:ident 1 * $unit3:ident 1 } => { ... };
    { $exact:tt $with_approx:tt $with_correlation:tt, $unit1:ident 2 == $unit2:ident 1 * $unit3:ident 2 } => { ... };
    { $exact:tt $with_approx:tt $with_correlation:tt, $unit1:ident 2 == $unit2:ident 2 * $unit3:ident 1 } => { ... };
    { $exact:tt $with_approx:tt $with_correlation:tt, $unit1:ident 3 == $unit2:ident 1 * $unit3:ident 3 } => { ... };
    { $exact:tt $with_approx:tt $with_correlation:tt, $unit1:ident 3 == $unit2:ident 3 * $unit3:ident 1 } => { ... };
    { $exact:tt $with_approx:tt $with_correlation:tt, $unit1:ident 1 == $unit2:ident 2 * __ 2 } => { ... };
    { $exact:tt $with_approx:tt $with_correlation:tt, $unit1:ident 1 == $unit2:ident 2 * $unit3:ident 2 } => { ... };
    { $exact:tt $with_approx:tt $with_correlation:tt, $unit1:ident 1 == $unit2:ident 3 * __ 3 } => { ... };
    { $exact:tt $with_approx:tt $with_correlation:tt, $unit1:ident 1 == $unit2:ident 3 * $unit3:ident 3 } => { ... };
    { $exact:tt $with_approx:tt $with_correlation:tt, $unit1:ident 1 == $unit2:ident 2 X __ 2 } => { ... };
    { $exact:tt $with_approx:tt $with_correlation:tt, $unit1:ident 1 == $unit2:ident 2 X $unit3:ident 2 } => { ... };
    { $exact:tt $with_approx:tt $with_correlation:tt, $unit1:ident 3 == $unit2:ident 3 X __ 3 } => { ... };
    { $exact:tt $with_approx:tt $with_correlation:tt, $unit1:ident 3 == $unit2:ident 3 X $unit3:ident 3 } => { ... };
}