Nested substitution rules in instructions aren't expanded

If calls to substitution rules (in actual instructions) are nested within each other, expand_subst will only expand the outermost rule. All rules are removed from the substitution rule list, but the calls to the nested rules remain in the kernel

import loopy as lp

knl = lp.make_kernel(
    "{[i]: 0<=i<n}",
    """
    a(x) := 2 * x
    b(x) := x**2
    f[i] = b(a(i))    
    """
)

knl = lp.expand_subst(knl)
print(knl)

yields

---------------------------------------------------------------------------
KERNEL: loopy_kernel
---------------------------------------------------------------------------
ARGUMENTS:
f: type: <auto/runtime>, shape: (n), dim_tags: (N0:stride:1) aspace: global
n: ValueArg, type: <auto/runtime>
---------------------------------------------------------------------------
DOMAINS:
[n] -> { [i] : 0 <= i < n }
---------------------------------------------------------------------------
INAME IMPLEMENTATION TAGS:
i: None
---------------------------------------------------------------------------
INSTRUCTIONS:
for i
  f[i] = a(i)**2  {id=insn}
end i
---------------------------------------------------------------------------

Trying to run the kernel fails catastrophically at clBuildProgram, of course.

Note that if the nesting is done within the actual definition of the rule, the expansion is correct, e.g. the following instruction body correctly:

a(x) := 2 * x
b(x) := a(x)**2
f[i] = b(i)