feat(flomni): add remove-only sample change via ftransfer_sample_change(-1)

Passing -1 stows the sample currently in the sample stage into a free
tray slot without mounting a replacement, matching the old control
system's sample_change(-1) behavior. Existing swap-path logic is
unchanged.

csaxs_bec/bec_ipython_client/plugins/flomni/flomni.py

@@ -839,12 +839,45 @@ class FlomniSampleTransferMixin:
         if sample_in_gripper:
             raise FlomniError("There is already a sample in the gripper. Aborting.")
 
-        self.check_position_is_valid(new_sample_position)
+        if new_sample_position == -1:
+            # Remove-only: stow the sample currently in the sample stage into a
+            # free tray slot and stop there -- no new sample is mounted.
+            sample_in_sample_stage = dev.flomni_samples.is_sample_slot_used(0)
+            if not sample_in_sample_stage:
+                raise FlomniError("There is no sample in the sample stage to remove. Aborting.")
 
-        if new_sample_position == 0:
-            raise FlomniError(
-                "The new sample to place cannot be the sample in the sample stage. Aborting."
-            )
+            empty_slots = []
+            for j in range(1, 20):
+                if not dev.flomni_samples.is_sample_slot_used(j):
+                    empty_slots.append(j)
+            if not empty_slots:
+                raise FlomniError("There are no empty slots available. Aborting.")
+
+            print(f"The following slots are empty: {empty_slots}.")
+
+            while True:
+                user_input = input(f"Where shall I put the sample? Default: [{empty_slots[0]}] ")
+
+                if user_input.strip() == "":
+                    user_input = empty_slots[0]
+                    break
+
+                try:
+                    user_input = int(user_input)
+                    if user_input not in empty_slots:
+                        raise ValueError
+                    break
+                except ValueError:
+                    print("Please specify a valid number.")
+                    continue
+
+            self.check_position_is_valid(user_input)
+
+            self.ftransfer_get_sample(0)
+            self.ftransfer_put_sample(user_input)
+            return
+
+        self.check_position_is_valid(new_sample_position)
 
         #        sample_placed = getattr(
         #            dev.flomni_samples.sample_placed, f"sample{new_sample_position}"
@@ -855,6 +888,11 @@ class FlomniSampleTransferMixin:
                 f"There is currently no sample in position [{new_sample_position}]. Aborting."
             )
 
+        if new_sample_position == 0:
+            raise FlomniError(
+                "The new sample to place cannot be the sample in the sample stage. Aborting."
+            )
+
         #        sample_in_sample_stage = dev.flomni_samples.sample_placed.sample0.get()
         sample_in_sample_stage = dev.flomni_samples.is_sample_slot_used(0)
         if sample_in_sample_stage:

docs/user/ptychography/flomni.md

@@ -24,6 +24,10 @@ You will be asked to enter a sample name.
 To load a new sample in the sample stage, in principle only one command is needed  
 `flomni.ftransfer_sample_change(<position new sample>)`  
 You will be asked where the previous sample should go with a suggestion for an empty position in the tray.
+
+To remove the current sample from the sample stage __without__ mounting a new one, use  
+`flomni.ftransfer_sample_change(-1)`  
+You will be asked where to stow the removed sample, same as above.
 Other commands:  
 `ftransfer_tray_in /_out	 (not yet implemented).`