The above command implies that 'F' is a symbolic function. Hence, the command
does not give error.
However, 't' is defined as a symbolic variable, and so, 'DexpAt' is also a symbolic variable. As variables cannot take any input arguments, the below command gives an error.
For the above command to run without any error, you can define 'DexpAt' also as a symbolic function by changing
You can check your workspace to confirm if any symbol is a symbolic function or a symbolic variable.
For additional information on symbolic variables and functions, refer to the following link: