In the Natural Units environment, the global verify function is replaced by a verify function that converts any unevaluated arithmetic operators, equalities, or inequalities to their global equivalents.  The first two arguments are tested to check whether they are valid unit names.

For more information about this function, see the global verify help page.

verify(3.50000003 = 'a1', 3.499999997 = 'a1', 'float(100) = boolean');

$\mathrm{true}$

with(Units[Natural]):

:-verify(3.50000003 = 'a1', 3.499999997 = 'a1', 'float(100) = boolean');  # unexpectedly false

$\mathrm{false}$

verify(3.50000003 = 'a1', 3.499999997 = 'a1', 'float(100) = boolean');

$\mathrm{true}$

:-verify(m, 1250/381*ft, 'units');  # this returns false as 'm' is not interpreted as a meter

$\mathrm{false}$

verify(m, 1250/381*ft, 'units');

$\mathrm{true}$