Use anaconda 2 after anaconda 3 upgrade

Question

I am trying to do a pip install within anaconda, i.e. with an environment activated. When I had previously installed the package with anaconda 2, it worked fine. When I do the same install with anaconda 3, I get the error reproduced below. Is there a way to either:

(a) use anaconda 2 even with anaconda 3 installed? (b) downgrade to anaconda 2, completely, and start from scratch? (c) avoid this C++ error?

Here is the error, in case it helps:

(lm_1b) vpn-165-124-160-218:~ adamg$ pip install https://github.com/kpu/kenlm/archive/master.zip
Collecting https://github.com/kpu/kenlm/archive/master.zip
  Downloading https://github.com/kpu/kenlm/archive/master.zip (526kB)
    100% |████████████████████████████████| 532kB 1.1MB/s
Building wheels for collected packages: kenlm
  Running setup.py bdist_wheel for kenlm ... error
  Complete output from command /Users/adamg/anaconda3/envs/lm_1b/bin/python -u -c "import setuptools, tokenize;__file__='/private/var/folders/2l/hd8b8vx566ld71lfd8hjglbc0000gn/T/pip-req-build-bkyn5o51/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" bdist_wheel -d /private/var/folders/2l/hd8b8vx566ld71lfd8hjglbc0000gn/T/pip-wheel-vbsheajs --python-tag cp36:
  running bdist_wheel
  running build
  running build_ext
  building 'kenlm' extension
  creating build
  creating build/temp.macosx-10.9-x86_64-3.6
  creating build/temp.macosx-10.9-x86_64-3.6/util
  creating build/temp.macosx-10.9-x86_64-3.6/lm
  creating build/temp.macosx-10.9-x86_64-3.6/util/double-conversion
  creating build/temp.macosx-10.9-x86_64-3.6/python
  clang -Wno-unused-result -Wsign-compare -Wunreachable-code -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -I/Users/adamg/anaconda3/envs/lm_1b/include -arch x86_64 -I/Users/adamg/anaconda3/envs/lm_1b/include -arch x86_64 -I. -I/Users/adamg/anaconda3/envs/lm_1b/include/python3.6m -c util/pool.cc -o build/temp.macosx-10.9-x86_64-3.6/util/pool.o -O3 -DNDEBUG -DKENLM_MAX_ORDER=6 -std=c++11
  In file included from util/pool.cc:1:
  In file included from ./util/pool.hh:4:
  /Users/adamg/anaconda3/envs/lm_1b/include/c++/v1/cassert:21:10: fatal error: 'assert.h' file not found
  #include <assert.h>
           ^~~~~~~~~~
  1 error generated.
  error: command 'clang' failed with exit status 1

  ----------------------------------------
  Failed building wheel for kenlm
  Running setup.py clean for kenlm
Failed to build kenlm
Installing collected packages: kenlm
  Running setup.py install for kenlm ... error
    Complete output from command /Users/adamg/anaconda3/envs/lm_1b/bin/python -u -c "import setuptools, tokenize;__file__='/private/var/folders/2l/hd8b8vx566ld71lfd8hjglbc0000gn/T/pip-req-build-bkyn5o51/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /private/var/folders/2l/hd8b8vx566ld71lfd8hjglbc0000gn/T/pip-record-74c181ep/install-record.txt --single-version-externally-managed --compile:
    running install
    running build
    running build_ext
    building 'kenlm' extension
    creating build
    creating build/temp.macosx-10.9-x86_64-3.6
    creating build/temp.macosx-10.9-x86_64-3.6/util
    creating build/temp.macosx-10.9-x86_64-3.6/lm
    creating build/temp.macosx-10.9-x86_64-3.6/util/double-conversion
    creating build/temp.macosx-10.9-x86_64-3.6/python
    clang -Wno-unused-result -Wsign-compare -Wunreachable-code -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -I/Users/adamg/anaconda3/envs/lm_1b/include -arch x86_64 -I/Users/adamg/anaconda3/envs/lm_1b/include -arch x86_64 -I. -I/Users/adamg/anaconda3/envs/lm_1b/include/python3.6m -c util/pool.cc -o build/temp.macosx-10.9-x86_64-3.6/util/pool.o -O3 -DNDEBUG -DKENLM_MAX_ORDER=6 -std=c++11
    In file included from util/pool.cc:1:
    In file included from ./util/pool.hh:4:
    /Users/adamg/anaconda3/envs/lm_1b/include/c++/v1/cassert:21:10: fatal error: 'assert.h' file not found
    #include <assert.h>
             ^~~~~~~~~~
    1 error generated.
    error: command 'clang' failed with exit status 1

    ----------------------------------------
Command "/Users/adamg/anaconda3/envs/lm_1b/bin/python -u -c "import setuptools, tokenize;__file__='/private/var/folders/2l/hd8b8vx566ld71lfd8hjglbc0000gn/T/pip-req-build-bkyn5o51/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /private/var/folders/2l/hd8b8vx566ld71lfd8hjglbc0000gn/T/pip-record-74c181ep/install-record.txt --single-version-externally-managed --compile" failed with error code 1 in /private/var/folders/2l/hd8b8vx566ld71lfd8hjglbc0000gn/T/pip-req-build-bkyn5o51/