vary statement to extend values of existing variables causes crash

The vary statement is not working when used to extend the values of a variable already existing in a model. This problem affects the master branch, and occurs in both session and script CLI tools.

Interactive session

For the interactive session, the use of evaluated and non-evaluated variables seems to have no effect (whether the -r flag or %start magic have been used before or after vary is irrelevant as the error always occurs). If vary is attempted, a series of error messages (see below) is printed, the session crashes, and obviously no additional models are added to the group. In contrast, the use of vary statement for new variables works without problems, and all corresponding models are added to the group.

The error can be reproduced by evaluating the following model (vary_test.vm) in a session:

vara = 1.0
varb = 0.5
varc = 0.25

eq1 = varb / vara

Then the model should be executed and loaded into an interactive session:

~>$ texts script -m workflow -f vary_test.vm
program UUID: some_sample_UUID
program output: >>>
n.c.
<<<

~>$ texts session -w -u some_sample_UUID
Input > 
Input > vary ((vara: 2.0, 3.0))
Traceback (most recent call last):
[...]
  File "/hkfs/home/project/hk-project-consulting/an9294/python_VirtEnvs/vrel-br220/lib64/python3.9/site-packages/pandas/core/reshape/merge.py", line 1508, in _maybe_coerce_merge_keys
    raise ValueError(msg)
ValueError: You are trying to merge on pint[dimensionless] and object columns for key 'varc'. If you wish to proceed you should use pd.concat

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
[...]
  File "/hkfs/home/project/hk-project-consulting/an9294/python_VirtEnvs/vrel-br220/lib/python3.9/site-packages/virtmat/language/utilities/errors.py", line 337, in wrapper
    raise RuntimeError('non-handled exception') from err
RuntimeError: non-handled exception

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
[...]
  File "/hkfs/home/project/hk-project-consulting/an9294/python_VirtEnvs/vrel-br220/lib/python3.9/site-packages/virtmat/language/utilities/errors.py", line 337, in wrapper
    raise RuntimeError('non-handled exception') from err
RuntimeError: non-handled exception

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
[...]
  File "/hkfs/home/project/hk-project-consulting/an9294/python_VirtEnvs/vrel-br220/lib/python3.9/site-packages/virtmat/language/utilities/errors.py", line 337, in wrapper
    raise RuntimeError('non-handled exception') from err
RuntimeError: non-handled exception
~> $

A similar message is printed when vary is used for any of the existing variables in the model. Using the same model (and UUID) as starting point, the following example vary` of a new variable works:

~>$ texts session -w -u some_sample_UUID
Input > vary ((vard: 'a', 'b'))
Input > %uuid
uuids: '0402dcfa6dd748e2a83f8378f9607ba2' ('0402dcfa6dd748e2a83f8378f9607ba2', '7c3e91c8234e424ca4e8ddf3d6a135e7')
Input > %hist
WAITING    2024-08-07T17:34:12+02:00 eq1 = varb / vara
WAITING    2024-08-07T17:34:12+02:00 varc = 0.25
WAITING    2024-08-07T17:34:12+02:00 varb = 0.5
WAITING    2024-08-07T17:34:12+02:00 vara = 1.0
WAITING    2024-08-07T17:37:27+02:00 vard = 'a'
Input > %vary
vary: ((vard: 'a', 'b'), ('%uuid': '0402dcfa6dd748e2a83f8378f9607ba2', '7c3e91c8234e424ca4e8ddf3d6a135e7'))
Input > %bye

Script tool

The issue can be reproduced using thie following variant of the previous script, which includes the vary statement:

vara = 1.0
varb = 0.5
varc = 0.25

vary ((vara: 2.0, 3.0))
#vary ((vard: 'a', 'b'))

eq1 = varb / vara

If the model is executed as provided above, the following message is printed and the program crashes:

~>$ texts script -m workflow -f vary_test2.vm
Initialization error: None:1:64 --> vara = 2.0 <--
Repeated initialization of "vara"

However, if the new variable vard is used as parameter for vary, then the program works as expected:

~>$ texts script -m workflow -f vary_tst2.vm
program UUID: some_other_UUID
program output: >>>
n.c.
<<<
~>$ texts session -w -u some_other_UUID
Input > %uuid
uuids: '723bbcea62ca4c3dbc96870e4c062b2c' ('723bbcea62ca4c3dbc96870e4c062b2c', '066a6f3460a94086b013cdf081abb39b')
Input > %vary
vary: ((vard: 'a', 'b'), ('%uuid': '723bbcea62ca4c3dbc96870e4c062b2c', '066a6f3460a94086b013cdf081abb39b'))
Input > %hist
WAITING    2024-08-07T18:08:57+02:00 vard = 'a'
WAITING    2024-08-07T18:08:57+02:00 eq1 = varb / vara
WAITING    2024-08-07T18:08:57+02:00 varc = 0.25
WAITING    2024-08-07T18:08:57+02:00 varb = 0.5
WAITING    2024-08-07T18:08:57+02:00 vara = 1.0
Input >%bye

Strangely, the error in the session and script seem to originate from different things:

session: ValueError: You are trying to merge on pint[dimensionless] and object columns for key 'varc'
script: Repeated initialization of "vara"