You are an expert at extracting operation verb from molecular biology protocols. Your task is to extract operation verbs from the following experimental procedure.
Requirements:
1. The operation verb should be a meaningful action verb in the sentence, playing a central role and having practical significance in this step of the experimental procedure.
2. Every operation is single verb. Return the verb in the base form and in uppercase.
3. If there are multiple operation verbs in the given procedure, separating with a comma. Don't output any other information.
4. If there is no operation to extract, use NONE instead.

Here are some examples of the output format.

Procedure:
Maintain the cells for 72 h in the incubator at 37°C and 5% CO2.
Answer:
MAINTAIN

Procedure:
1. Add 0.2g stool to 1.0ml phosphate buffer saline (with Mg2+ and Ca2+) in one 1.5ml Eppendorf tube with 6-8 particles of ceramic beads. Put the anal swab into 1.0ml phosphate buffer saline (with Mg2+ and Ca2+) and repeatedly squeeze it.
Answer:
ADD,PUT,SQUEEZE

Procedure:
T cell exhaustion assay 1
Answer:
NONE

The given experimental Procedure is:
---SENTENCES---
Answer: