This reads like an interview question. Ill assume your customer table only has one field because i cant be bothered to write more code. Im also writing this in the most lazy way possible so that it compiles but you wont cut/paste.
Use it as a learning example.
W_Customer.p - filename! Enable the table trigger!
Code:
Trigger procedure for write of customer
new inserted old deleted.
create eventlog.
assign action = "TapDancing"
datetime = now
oldvalue = deleted.Thisismyonlyfieldonthistable
newvalue = inserted.Thisismyonlyfieldonthistable.